feat(06-01): Implement workout swap/rotation system - API, DB, frontend

- Add workout_swaps table migration (007_add_workout_swap_tracking.sql)
- Implement 4 API endpoints: POST /swap, DELETE /undo, GET /swaps, GET /available
- Add request validation, error handling, user isolation, muscle group checks
- Create SwapWorkoutModal React component with modal UI
- Integrate swap functionality into WorkoutPage
- Add proper styling for swap modal
- All endpoints require authentication
- Database migration includes performance indexes

db/migrations/007_add_workout_swap_tracking.sql

@@ -0,0 +1,21 @@
+-- Track which exercises were swapped
+CREATE TABLE IF NOT EXISTS workout_swaps (
+    id SERIAL PRIMARY KEY,
+    user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    program_exercise_id INTEGER NOT NULL REFERENCES program_exercises(id) ON DELETE CASCADE,
+    from_exercise_id INTEGER NOT NULL REFERENCES exercises(id) ON DELETE CASCADE,
+    to_exercise_id INTEGER NOT NULL REFERENCES exercises(id) ON DELETE CASCADE,
+    swap_date DATE NOT NULL,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
+
+-- Add reference in workout_logs to track origin
+ALTER TABLE workout_logs
+    ADD COLUMN IF NOT EXISTS swapped_from_id INTEGER REFERENCES workout_logs(id) ON DELETE SET NULL,
+    ADD COLUMN IF NOT EXISTS swap_history_id INTEGER REFERENCES workout_swaps(id) ON DELETE SET NULL;
+
+-- Indexes for performance
+CREATE INDEX IF NOT EXISTS idx_workout_swaps_user_date ON workout_swaps(user_id, swap_date);
+CREATE INDEX IF NOT EXISTS idx_workout_swaps_exercise ON workout_swaps(program_exercise_id);
+CREATE INDEX IF NOT EXISTS idx_workout_logs_swapped_from ON workout_logs(swapped_from_id);
+CREATE INDEX IF NOT EXISTS idx_workout_logs_swap_history ON workout_logs(swap_history_id);