Add lessons Phase 2: schedule slots with conflict detection

Recurring weekly time slots linking instructors to lesson types.
Includes day/time overlap detection, instructor and day-of-week
filtering, and 17 new integration tests (40 total lessons tests).

packages/backend/src/db/migrations/meta/_journal.json

@@ -204,6 +204,13 @@
       "when": 1774880000000,
       "tag": "0028_lessons_foundation",
       "breakpoints": true
+    },
+    {
+      "idx": 29,
+      "version": "7",
+      "when": 1774890000000,
+      "tag": "0029_schedule_slots",
+      "breakpoints": true
     }
   ]
 }