feat(02-02): add DELETE /api/logs endpoint to backend

- DELETE /api/logs accepts user_id, program_exercise_id, date, set_number in request body
- Deletes matching workout_logs row by composite key
- Returns 200 + deleted id on success, 404 if row not found
- Consistent with existing POST /api/logs (no auth middleware, user_id from body)

backend/src/index.js

@@ -328,6 +328,27 @@ app.post('/api/logs', async (req, res) => {
   }
 });
 
+// Delete a specific set log
+app.delete('/api/logs', async (req, res) => {
+  try {
+    const { user_id, program_exercise_id, date, set_number } = req.body;
+
+    const result = await pool.query(
+      'DELETE FROM workout_logs WHERE user_id = $1 AND program_exercise_id = $2 AND date = $3 AND set_number = $4 RETURNING id',
+      [user_id, program_exercise_id, date, set_number]
+    );
+
+    if (result.rows.length === 0) {
+      return res.status(404).json({ error: 'Log not found' });
+    }
+
+    res.json({ deleted: result.rows[0].id });
+  } catch (err) {
+    console.error('Error deleting log:', err);
+    res.status(500).json({ error: 'Database error' });
+  }
+});
+
 // Calculate suggested weight based on progression
 app.get('/api/progression/:programExerciseId', async (req, res) => {
   try {