docs: update changelog

docs/database-schema-readme.md

@@ -15,10 +15,10 @@ This document provides a comprehensive overview of the Mila Membership Managemen
 
 | Metric | Count |
 |--------|-------|
-| **Tables** | 9 |
+| **Tables** | 11 |
 | **Domains** | 4 (Accounts, Membership, MembershipFees, Authorization) |
-| **Relationships** | 7 |
-| **Indexes** | 20+ |
+| **Relationships** | 9 |
+| **Indexes** | 25+ |
 | **Triggers** | 1 (Full-text search) |
 
 ## Tables Overview
@@ -77,6 +77,23 @@ This document provides a comprehensive overview of the Mila Membership Managemen
   - Membership fee default settings
   - Environment variable support for club name
 
+#### `groups`
+- **Purpose:** Group definitions for organizing members
+- **Rows (Estimated):** Low (typically 5-20 groups per club)
+- **Key Features:**
+  - Unique group names (case-insensitive)
+  - URL-friendly slugs (auto-generated, immutable)
+  - Optional descriptions
+  - Many-to-many relationship with members
+
+#### `member_groups`
+- **Purpose:** Join table for many-to-many relationship between members and groups
+- **Rows (Estimated):** Medium to High (multiple groups per member)
+- **Key Features:**
+  - Unique constraint on (member_id, group_id)
+  - CASCADE delete on both sides
+  - Efficient indexes for queries
+
 ### Authorization Domain
 
 #### `roles`
@@ -100,6 +117,10 @@ Member (1) → (N) MembershipFeeCycles
                     ↓
             MembershipFeeType (1)
 
+Member (N) ←→ (N) Group
+       ↓              ↓
+  MemberGroups (N)  MemberGroups (N)
+
 Settings (1) → MembershipFeeType (0..1)
 ```
 
@@ -145,6 +166,12 @@ Settings (1) → MembershipFeeType (0..1)
    - Settings can reference a default fee type
    - `ON DELETE SET NULL` - if fee type is deleted, setting is cleared
 
+9. **Member ↔ Group (N:N via MemberGroup)**
+   - Many-to-many relationship through `member_groups` join table
+   - `ON DELETE CASCADE` on both sides - removing member/group removes associations
+   - Unique constraint on (member_id, group_id) prevents duplicates
+   - Groups searchable via member search vector
+
 ## Important Business Rules
 
 ### Email Synchronization
@@ -509,7 +536,7 @@ mix run priv/repo/seeds.exs
 
 ---
 
-**Last Updated:** 2026-01-13  
-**Schema Version:** 1.4  
+**Last Updated:** 2026-01-27  
+**Schema Version:** 1.5  
 **Database:** PostgreSQL 17.6 (dev) / 16 (prod)