refactor: improve cycle generation code quality and documentation

- Remove Process.sleep calls from integration tests (tests run synchronously in SQL sandbox)
- Improve error handling: membership_fee_type_not_found now returns changeset error instead of just logging
- Clarify partial_failure documentation: successful_cycles are not persisted on rollback
- Update documentation: joined_at → join_date, left_at → exit_date
- Document PostgreSQL advisory locks per member (not whole table lock)
- Document gap handling: explicitly deleted cycles are not recreated

docs/membership-fee-architecture.md

@@ -153,8 +153,8 @@ lib/
 
 **Existing Fields Used:**
 
-- `joined_at` - For calculating membership fee start
-- `left_at` - For limiting cycle generation
+- `join_date` - For calculating membership fee start
+- `exit_date` - For limiting cycle generation
 - These fields must remain member fields and should not be replaced by custom fields in the future
 
 ### Settings Integration
@@ -186,8 +186,9 @@ lib/
 
 - Calculate which cycles should exist for a member
 - Generate missing cycles
-- Respect membership_fee_start_date and left_at boundaries
+- Respect membership_fee_start_date and exit_date boundaries
 - Skip existing cycles (idempotent)
+- Use PostgreSQL advisory locks per member to prevent race conditions
 
 **Triggers:**
 
@@ -199,17 +200,20 @@ lib/
 **Algorithm Steps:**
 
 1. Retrieve member with membership fee type and dates
-2. Determine first cycle start (based on membership_fee_start_date)
-3. Calculate all cycle starts from first to today (or left_at)
-4. Query existing cycles for member
-5. Generate missing cycles with current membership fee type's amount
-6. Insert new cycles (batch operation)
+2. Determine generation start point:
+   - If NO cycles exist: Start from `membership_fee_start_date` (or calculated from `join_date`)
+   - If cycles exist: Start from the cycle AFTER the last existing one
+3. Generate all cycle starts from the determined start point to today (or `exit_date`)
+4. Create new cycles with current membership fee type's amount
+5. Use PostgreSQL advisory locks per member to prevent race conditions
 
 **Edge Case Handling:**
 
-- If membership_fee_start_date is NULL: Calculate from joined_at + global setting
-- If left_at is set: Stop generation at left_at
+- If membership_fee_start_date is NULL: Calculate from join_date + global setting
+- If exit_date is set: Stop generation at exit_date
 - If membership fee type changes: Handled separately by regeneration logic
+- **Gap Handling:** If cycles were explicitly deleted (gaps exist), they are NOT recreated.
+  The generator always continues from the cycle AFTER the last existing cycle, regardless of gaps.
 
 ### Calendar Cycle Calculations
 
@@ -381,7 +385,7 @@ lib/
 **AC-M-1:** Member has membership_fee_type_id field (NOT NULL with default)  
 **AC-M-2:** Member has membership_fee_start_date field (nullable)  
 **AC-M-3:** New members get default membership fee type from global setting  
-**AC-M-4:** membership_fee_start_date auto-set based on joined_at and global setting  
+**AC-M-4:** membership_fee_start_date auto-set based on join_date and global setting  
 **AC-M-5:** Admin can manually override membership_fee_start_date  
 **AC-M-6:** Cannot change to membership fee type with different interval (MVP)
 
@@ -391,7 +395,7 @@ lib/
 **AC-CG-2:** Cycles generated when member created (via change hook)  
 **AC-CG-3:** Scheduled job generates missing cycles daily  
 **AC-CG-4:** Generation respects membership_fee_start_date  
-**AC-CG-5:** Generation stops at left_at if member exited  
+**AC-CG-5:** Generation stops at exit_date if member exited  
 **AC-CG-6:** Generation is idempotent (skips existing cycles)  
 **AC-CG-7:** Cycles align to calendar boundaries (1st of month/quarter/half/year)  
 **AC-CG-8:** Amount comes from membership_fee_type at generation time
@@ -472,8 +476,9 @@ lib/
 - Correct cycle_start calculation for all interval types
 - Correct cycle count from start to end date
 - Respects membership_fee_start_date boundary
-- Respects left_at boundary
+- Respects exit_date boundary
 - Skips existing cycles (idempotent)
+- Does not fill gaps when cycles were deleted
 - Handles edge dates (year boundaries, leap years)
 
 **Calendar Cycles Tests:**