Add lessons Phase 6: lesson plans with curriculum tracking

Structured lesson plans with nested sections and items per enrollment.
Deep create in one request, one-active-per-enrollment constraint,
auto-set startedDate/masteredDate on status transitions, progress %
calculation (skipped items excluded). 8 new tests (84 total).

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

@@ -232,6 +232,13 @@
       "when": 1774920000000,
       "tag": "0032_grading_scales",
       "breakpoints": true
+    },
+    {
+      "idx": 33,
+      "version": "7",
+      "when": 1774930000000,
+      "tag": "0033_lesson_plans",
+      "breakpoints": true
     }
   ]
 }