Add Phase 4b: instructor blocked dates, store closures, and substitute instructors

- New tables: instructor_blocked_date, store_closure (migration 0034)
- substitute_instructor_id column added to lesson_session
- Session generation skips blocked instructor dates and store closure periods
- Substitute assignment validates sub is not blocked and has no conflicting slot
- Routes: POST/GET/DELETE /instructors/:id/blocked-dates, POST/GET/DELETE /store-closures
- 15 new integration tests covering blocked dates, store closures, and sub validation

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

@@ -239,6 +239,13 @@
       "when": 1774930000000,
       "tag": "0033_lesson_plans",
       "breakpoints": true
+    },
+    {
+      "idx": 34,
+      "version": "7",
+      "when": 1774940000000,
+      "tag": "0034_blocked_dates",
+      "breakpoints": true
     }
   ]
 }