feat(phase-4): Backend API for custom workouts

- Add custom_workouts and custom_workout_exercises tables (schema)
- New endpoints:
  - GET /api/exercises - List all exercises for picker
  - POST /api/custom-workouts - Fork program workout
  - GET /api/custom-workouts - List user's custom workouts
  - GET /api/custom-workouts/:id - Get workout with exercises
  - PUT /api/custom-workouts/:id - Update workout exercises
  - DELETE /api/custom-workouts/:id - Delete custom workout
- Updated endpoints for source_type support:
  - GET /api/logs - Filter by source_type and custom_workout_id
  - POST /api/logs - Save with source_type and custom_workout_id
  - DELETE /api/logs - Support custom workout log deletion
- Adds Phase 4 planning overview

Completes: 04-01-schema-migration, 04-02-backend-api
Next: 04-03-frontend-workout-edit

.pm-checkpoint.json

@@ -1,8 +1,9 @@
 {
-  "lastRun": "2026-02-28T23:45:00+01:00",
+  "lastRun": "2026-03-01T04:00:00+01:00",
   "status": "completed",
-  "tasksCompleted": ["emoji-replacement", "alternative-modal", "workout-page-ux-redish", "chat-onboarding", "03-01-login-onboarding-polish", "03-02-dashboard-polish", "03-03-workout-experience-polish"],
-  "activeTask": null,
-  "nextTask": null,
-  "notes": "03-03 Workout Experience Polish complete (commit f6b1379). Phase 3 complete: login/onboarding, dashboard, and workout experience all polished."
+  "phase": "04-workout-modification",
+  "activeTask": "04-02-backend-api",
+  "tasksCompleted": ["01-input-ux", "02-flexible-sets", "03-design-polish", "04-01-schema-migration", "04-02-backend-api"],
+  "nextTask": "04-03-frontend-workout-edit",
+  "notes": "Backend API complete for custom workouts. Added 6 new endpoints + updated 3 log endpoints with source_type support. Next: Frontend edit UI."
 }