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/shared/src/schemas/lessons.schema.ts

@@ -106,3 +106,28 @@ export const LessonSessionUpdateSchema = z.object({
   actualEndTime: opt(z.string().regex(/^\d{2}:\d{2}$/, 'Must be HH:MM format')),
 })
 export type LessonSessionUpdateInput = z.infer<typeof LessonSessionUpdateSchema>
+
+// --- Grading Scale schemas ---
+
+export const GradingScaleLevelInput = z.object({
+  value: z.string().min(1).max(50),
+  label: z.string().min(1).max(255),
+  numericValue: z.coerce.number().int().min(0).max(100),
+  colorHex: opt(z.string().regex(/^#[0-9A-Fa-f]{6}$/, 'Must be #RRGGBB format')),
+  sortOrder: z.coerce.number().int(),
+})
+
+export const GradingScaleCreateSchema = z.object({
+  name: z.string().min(1).max(255),
+  description: opt(z.string()),
+  isDefault: z.boolean().default(false),
+  levels: z.array(GradingScaleLevelInput).min(1),
+})
+export type GradingScaleCreateInput = z.infer<typeof GradingScaleCreateSchema>
+
+export const GradingScaleUpdateSchema = z.object({
+  name: z.string().min(1).max(255).optional(),
+  description: opt(z.string()),
+  isDefault: z.boolean().optional(),
+})
+export type GradingScaleUpdateInput = z.infer<typeof GradingScaleUpdateSchema>