Add lessons Phase 5: grading scales with nested levels

Custom grading scales with ordered levels (value, label, numeric score,
color). Supports one-default-per-store constraint, deep create with
nested levels, lookup endpoint for dropdowns, and search/pagination.
12 new tests (76 total lessons tests).

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

@@ -225,6 +225,13 @@
       "when": 1774910000000,
       "tag": "0031_lesson_sessions",
       "breakpoints": true
+    },
+    {
+      "idx": 32,
+      "version": "7",
+      "when": 1774920000000,
+      "tag": "0032_grading_scales",
+      "breakpoints": true
     }
   ]
 }