docs(auth): document User policies and bypass pattern

Add bypass vs HasPermission pattern documentation
Update architecture and implementation plan docs

docs/policy-bypass-vs-haspermission.md

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+# Policy Pattern: Bypass vs. HasPermission
+
+**Date:** 2026-01-22  
+**Status:** Implemented and Tested  
+**Applies to:** User Resource, Member Resource
+
+---
+
+## Summary
+
+For filter-based permissions (`scope :own`, `scope :linked`), we use a **two-tier authorization pattern**:
+
+1. **Bypass with `expr()` for READ operations** - Handles list queries via auto_filter
+2. **HasPermission for UPDATE/CREATE/DESTROY** - Uses scope from PermissionSets when record is present
+
+This pattern ensures that the scope concept in PermissionSets is actually used and not redundant.
+
+---
+
+## The Problem
+
+### Initial Assumption (INCORRECT)
+
+> "No separate Own Credentials Bypass needed, as all permission sets already have User read/update :own. HasPermission with scope :own handles this correctly."
+
+This assumption was based on the idea that `HasPermission` returning `{:filter, expr(...)}` would automatically trigger Ash's `auto_filter` for list queries.
+
+### Reality
+
+**When HasPermission returns `{:filter, expr(...)}`:**
+
+1. `strict_check` is called first
+2. For list queries (no record yet), `strict_check` returns `{:ok, false}`
+3. Ash **STOPS** evaluation and does **NOT** call `auto_filter`
+4. Result: List queries fail with empty results ❌
+
+**Example:**
+```elixir
+# This FAILS for list queries:
+policy action_type([:read, :update]) do
+  authorize_if Mv.Authorization.Checks.HasPermission
+end
+
+# User tries to list all users:
+Ash.read(User, actor: user)
+# Expected: Returns [user] (filtered to own record)
+# Actual: Returns [] (empty list)
+```
+
+---
+
+## The Solution
+
+### Pattern: Bypass for READ, HasPermission for UPDATE
+
+**User Resource Example:**
+
+```elixir
+policies do
+  # Bypass for READ (handles list queries via auto_filter)
+  bypass action_type(:read) do
+    description "Users can always read their own account"
+    authorize_if expr(id == ^actor(:id))
+  end
+  
+  # HasPermission for UPDATE (scope :own works with changesets)
+  policy action_type([:read, :create, :update, :destroy]) do
+    description "Check permissions from user's role and permission set"
+    authorize_if Mv.Authorization.Checks.HasPermission
+  end
+end
+```
+
+**Why This Works:**
+
+| Operation | Record Available? | Method | Result |
+|-----------|-------------------|--------|--------|
+| **READ (list)** | ❌ No | `bypass` with `expr()` | Ash applies expr as SQL WHERE → ✅ Filtered list |
+| **READ (single)** | ✅ Yes | `bypass` with `expr()` | Ash evaluates expr → ✅ true/false |
+| **UPDATE** | ✅ Yes (changeset) | `HasPermission` with `scope :own` | strict_check evaluates record → ✅ Authorized |
+| **CREATE** | ✅ Yes (changeset) | `HasPermission` with `scope :own` | strict_check evaluates record → ✅ Authorized |
+| **DESTROY** | ✅ Yes | `HasPermission` with `scope :own` | strict_check evaluates record → ✅ Authorized |
+
+---
+
+## Why `scope :own` Is NOT Redundant
+
+### The Question
+
+> "If we use a bypass with `expr(id == ^actor(:id))` for READ, isn't `scope :own` in PermissionSets redundant?"
+
+### The Answer: NO! ✅
+
+**`scope :own` is ONLY used for operations where a record is present:**
+
+```elixir
+# PermissionSets.ex
+%{resource: "User", action: :read, scope: :own, granted: true},   # Not used (bypass handles it)
+%{resource: "User", action: :update, scope: :own, granted: true}, # USED by HasPermission ✅
+```
+
+**Test Proof:**
+
+```elixir
+# test/mv/accounts/user_policies_test.exs:82
+test "can update own email", %{user: user} do
+  new_email = "updated@example.com"
+  
+  # This works via HasPermission with scope :own (NOT via bypass)
+  {:ok, updated_user} =
+    user
+    |> Ash.Changeset.for_update(:update_user, %{email: new_email})
+    |> Ash.update(actor: user)
+  
+  assert updated_user.email == Ash.CiString.new(new_email)
+end
+# ✅ Test passes - proves scope :own is used!
+```
+
+---
+
+## Consistency Across Resources
+
+### User Resource
+
+```elixir
+# Bypass for READ list queries
+bypass action_type(:read) do
+  authorize_if expr(id == ^actor(:id))
+end
+
+# HasPermission for UPDATE (uses scope :own from PermissionSets)
+policy action_type([:read, :create, :update, :destroy]) do
+  authorize_if Mv.Authorization.Checks.HasPermission
+end
+```
+
+**PermissionSets:**
+- `own_data`, `read_only`, `normal_user`: `scope :own` for read/update
+- `admin`: `scope :all` for all operations
+
+### Member Resource
+
+```elixir
+# Bypass for READ list queries
+bypass action_type(:read) do
+  authorize_if expr(id == ^actor(:member_id))
+end
+
+# HasPermission for UPDATE (uses scope :linked from PermissionSets)
+policy action_type([:read, :create, :update, :destroy]) do
+  authorize_if Mv.Authorization.Checks.HasPermission
+end
+```
+
+**PermissionSets:**
+- `own_data`: `scope :linked` for read/update
+- `read_only`: `scope :all` for read (no update permission)
+- `normal_user`, `admin`: `scope :all` for all operations
+
+---
+
+## Technical Deep Dive
+
+### Why Does `expr()` in Bypass Work?
+
+**Ash treats `expr()` natively in two contexts:**
+
+1. **strict_check** (single record):
+   - Ash evaluates the expression against the record
+   - Returns true/false based on match
+
+2. **auto_filter** (list queries):
+   - Ash compiles the expression to SQL WHERE clause
+   - Applies filter directly in database query
+
+**Example:**
+```elixir
+bypass action_type(:read) do
+  authorize_if expr(id == ^actor(:id))
+end
+
+# For list query: Ash.read(User, actor: user)
+# Compiled SQL: SELECT * FROM users WHERE id = $1 (user.id)
+# Result: [user] ✅
+```
+
+### Why Doesn't HasPermission Trigger auto_filter?
+
+**HasPermission.strict_check logic:**
+
+```elixir
+def strict_check(actor, authorizer, _opts) do
+  # ...
+  case check_permission(...) do
+    {:filter, filter_expr} ->
+      if record do
+        # Evaluate filter against record
+        evaluate_filter_for_strict_check(filter_expr, actor, record, resource_name)
+      else
+        # No record (list query) - return false
+        # Ash STOPS here, does NOT call auto_filter
+        {:ok, false}
+      end
+  end
+end
+```
+
+**Why return false instead of :unknown?**
+
+We tested returning `:unknown`, but Ash's policy evaluation still didn't reliably call `auto_filter`. The `bypass` with `expr()` is the only consistent solution.
+
+---
+
+## Design Principles
+
+### 1. Consistency
+
+Both User and Member follow the same pattern:
+- Bypass for READ (list queries)
+- HasPermission for UPDATE/CREATE/DESTROY (with scope)
+
+### 2. Scope Concept Is Essential
+
+PermissionSets define scopes for all operations:
+- `:own` - User can access their own records
+- `:linked` - User can access linked records (e.g., their member)
+- `:all` - User can access all records (admin)
+
+**These scopes are NOT redundant** - they are used for UPDATE/CREATE/DESTROY.
+
+### 3. Bypass Is a Technical Workaround
+
+The bypass is not a design choice but a **technical necessity** due to Ash's policy evaluation behavior:
+- Ash doesn't call `auto_filter` when `strict_check` returns `false`
+- `expr()` in bypass is handled natively by Ash for both contexts
+- This is consistent with Ash's documentation and best practices
+
+---
+
+## Test Coverage
+
+### User Resource Tests
+
+**File:** `test/mv/accounts/user_policies_test.exs`
+
+**Coverage:**
+- ✅ 31 tests: 30 passing, 1 skipped
+- ✅ All 4 permission sets: `own_data`, `read_only`, `normal_user`, `admin`
+- ✅ READ operations (list and single) via bypass
+- ✅ UPDATE operations via HasPermission with `scope :own`
+- ✅ Admin operations via HasPermission with `scope :all`
+- ✅ AshAuthentication bypass (registration/login)
+- ✅ NoActor bypass (test environment)
+
+**Key Tests Proving Pattern:**
+
+```elixir
+# Test 1: READ list uses bypass (returns filtered list)
+test "list users returns only own user", %{user: user} do
+  {:ok, users} = Ash.read(Accounts.User, actor: user, domain: Mv.Accounts)
+  assert length(users) == 1  # Filtered to own user ✅
+  assert hd(users).id == user.id
+end
+
+# Test 2: UPDATE uses HasPermission with scope :own
+test "can update own email", %{user: user} do
+  {:ok, updated_user} =
+    user
+    |> Ash.Changeset.for_update(:update_user, %{email: "new@example.com"})
+    |> Ash.update(actor: user)
+  
+  assert updated_user.email  # Uses scope :own from PermissionSets ✅
+end
+
+# Test 3: Admin uses HasPermission with scope :all
+test "admin can update other users", %{admin: admin, other_user: other_user} do
+  {:ok, updated_user} =
+    other_user
+    |> Ash.Changeset.for_update(:update_user, %{email: "admin-changed@example.com"})
+    |> Ash.update(actor: admin)
+  
+  assert updated_user.email  # Uses scope :all from PermissionSets ✅
+end
+```
+
+---
+
+## Lessons Learned
+
+1. **Don't assume** that returning a filter from `strict_check` will trigger `auto_filter` - test it!
+2. **Bypass with `expr()` is necessary** for list queries with filter-based permissions
+3. **Scope concept is NOT redundant** - it's used for operations with records (UPDATE/CREATE/DESTROY)
+4. **Consistency matters** - following the same pattern across resources improves maintainability
+5. **Documentation is key** - explaining WHY the pattern exists prevents future confusion
+
+---
+
+## Future Considerations
+
+### If Ash Changes Policy Evaluation
+
+If a future version of Ash reliably calls `auto_filter` when `strict_check` returns `:unknown` or `{:filter, expr}`:
+
+1. We could **remove** the bypass for READ
+2. Keep only the HasPermission policy for all operations
+3. Update tests to verify the new behavior
+
+**However, for now (Ash 3.13.1), the bypass pattern is necessary and correct.**
+
+---
+
+## References
+
+- **Ash Policy Documentation**: [https://hexdocs.pm/ash/policies.html](https://hexdocs.pm/ash/policies.html)
+- **Implementation**: `lib/accounts/user.ex` (lines 271-315)
+- **Tests**: `test/mv/accounts/user_policies_test.exs`
+- **Architecture Doc**: `docs/roles-and-permissions-architecture.md`
+- **Permission Sets**: `lib/mv/authorization/permission_sets.ex`