Add Phase 7: grade history and session-plan item linking

- New tables: lesson_plan_item_grade_history (append-only), lesson_session_plan_item
- Grading an item updates current_grade_value and creates immutable history record
- Grading a not_started item auto-transitions it to in_progress
- Linking items to a session also auto-transitions not_started items
- Link operation is idempotent — re-linking same items produces no duplicates
- Endpoints: POST/GET /lesson-plan-items/:id/grades, GET /lesson-plan-items/:id/grade-history
- Endpoints: POST/GET /lesson-sessions/:id/plan-items
- 8 new integration tests

packages/backend/src/db/migrations/0035_grade_history.sql

@@ -0,0 +1,19 @@
+-- Phase 7: Grade history and session-plan item linking
+
+CREATE TABLE "lesson_plan_item_grade_history" (
+  "id" uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  "lesson_plan_item_id" uuid NOT NULL REFERENCES "lesson_plan_item"("id"),
+  "grading_scale_id" uuid REFERENCES "grading_scale"("id"),
+  "grade_value" varchar(50) NOT NULL,
+  "graded_by" uuid REFERENCES "user"("id"),
+  "session_id" uuid REFERENCES "lesson_session"("id"),
+  "notes" text,
+  "created_at" timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE TABLE "lesson_session_plan_item" (
+  "id" uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  "session_id" uuid NOT NULL REFERENCES "lesson_session"("id"),
+  "lesson_plan_item_id" uuid NOT NULL REFERENCES "lesson_plan_item"("id"),
+  "created_at" timestamptz NOT NULL DEFAULT now()
+);

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

@@ -246,6 +246,13 @@
       "when": 1774940000000,
       "tag": "0034_blocked_dates",
       "breakpoints": true
+    },
+    {
+      "idx": 35,
+      "version": "7",
+      "when": 1774950000000,
+      "tag": "0035_grade_history",
+      "breakpoints": true
     }
   ]
 }