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/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
     }
   ]
 }