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/0034_blocked_dates.sql

@@ -0,0 +1,21 @@
+-- Phase 4b: Instructor blocked dates, store closures, and substitute instructor on sessions
+
+ALTER TABLE "lesson_session"
+  ADD COLUMN "substitute_instructor_id" uuid REFERENCES "instructor"("id");
+
+CREATE TABLE "instructor_blocked_date" (
+  "id" uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  "instructor_id" uuid NOT NULL REFERENCES "instructor"("id"),
+  "start_date" date NOT NULL,
+  "end_date" date NOT NULL,
+  "reason" varchar(255),
+  "created_at" timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE TABLE "store_closure" (
+  "id" uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  "name" varchar(255) NOT NULL,
+  "start_date" date NOT NULL,
+  "end_date" date NOT NULL,
+  "created_at" timestamptz NOT NULL DEFAULT now()
+);