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/0032_grading_scales.sql

@@ -0,0 +1,22 @@
+-- Phase 5: Grading scales — custom grade definitions per store
+
+CREATE TABLE "grading_scale" (
+  "id" uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  "name" varchar(255) NOT NULL,
+  "description" text,
+  "is_default" boolean NOT NULL DEFAULT false,
+  "created_by" uuid REFERENCES "user"("id"),
+  "is_active" boolean NOT NULL DEFAULT true,
+  "created_at" timestamptz NOT NULL DEFAULT now(),
+  "updated_at" timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE TABLE "grading_scale_level" (
+  "id" uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  "grading_scale_id" uuid NOT NULL REFERENCES "grading_scale"("id"),
+  "value" varchar(50) NOT NULL,
+  "label" varchar(255) NOT NULL,
+  "numeric_value" integer NOT NULL,
+  "color_hex" varchar(7),
+  "sort_order" integer NOT NULL
+);

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
     }
   ]
 }