fix: remove ON CONFLICT clause after constraint removal

Migration 011 removed UNIQUE constraint (profile_id, date) to allow
multiple rest days per date, but INSERT still used ON CONFLICT.

Error: psycopg2.errors.InvalidColumnReference: there is no unique or
exclusion constraint matching the ON CONFLICT specification

Solution: Remove ON CONFLICT clause, use plain INSERT.
Multiple entries per date now allowed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/routers/rest_days.py

@@ -93,15 +93,11 @@ def create_rest_day(
     with get_db() as conn:
         cur = get_cursor(conn)
 
-        # Upsert by (profile_id, date)
+        # Insert (multiple entries per date allowed)
         cur.execute(
             """
             INSERT INTO rest_days (profile_id, date, rest_config, note, created_at)
             VALUES (%s, %s, %s, %s, CURRENT_TIMESTAMP)
-            ON CONFLICT (profile_id, date)
-            DO UPDATE SET
-                rest_config = EXCLUDED.rest_config,
-                note = EXCLUDED.note
             RETURNING id, profile_id, date, rest_config, note, created_at
             """,
             (pid, data.date, Json(config_dict), data.note)