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

db/migrations/004_add_custom_workouts.sql

@@ -0,0 +1,37 @@
+-- Migration 004: Add custom workout support
+-- Allows users to create personalized workout plans based on program days
+
+-- Custom workouts created by users
+CREATE TABLE IF NOT EXISTS custom_workouts (
+    id SERIAL PRIMARY KEY,
+    user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    name VARCHAR(255) NOT NULL,
+    description TEXT,
+    source_program_day_id INTEGER REFERENCES program_days(id) ON DELETE SET NULL,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
+
+-- Exercises within a custom workout
+CREATE TABLE IF NOT EXISTS custom_workout_exercises (
+    id SERIAL PRIMARY KEY,
+    custom_workout_id INTEGER NOT NULL REFERENCES custom_workouts(id) ON DELETE CASCADE,
+    exercise_id INTEGER NOT NULL REFERENCES exercises(id) ON DELETE CASCADE,
+    sets INTEGER NOT NULL DEFAULT 3,
+    reps_min INTEGER NOT NULL DEFAULT 8,
+    reps_max INTEGER NOT NULL DEFAULT 12,
+    rpe_target DECIMAL(3,1),
+    replaced_exercise_id INTEGER REFERENCES exercises(id) ON DELETE SET NULL,
+    order_index INTEGER NOT NULL DEFAULT 0,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
+
+-- Extend workout_logs to support custom workouts
+ALTER TABLE workout_logs
+    ADD COLUMN IF NOT EXISTS source_type VARCHAR(20) DEFAULT 'program' CHECK (source_type IN ('program', 'custom')),
+    ADD COLUMN IF NOT EXISTS custom_workout_id INTEGER REFERENCES custom_workouts(id) ON DELETE SET NULL;
+
+-- Indexes
+CREATE INDEX IF NOT EXISTS idx_custom_workouts_user ON custom_workouts(user_id);
+CREATE INDEX IF NOT EXISTS idx_custom_workout_exercises_workout ON custom_workout_exercises(custom_workout_id);
+CREATE INDEX IF NOT EXISTS idx_workout_logs_custom_workout ON workout_logs(custom_workout_id);