fix(05-01): add imageFilename to Zod validation schemas

- Root cause: Zod schemas for createItem and createCandidate did not
  include imageFilename field, so @hono/zod-validator silently stripped
  it from validated payloads before reaching the service layer
- Images uploaded successfully but filename was never persisted to DB
- Added imageFilename as optional string to both createItemSchema and
  createCandidateSchema

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/shared/schemas.ts

@@ -7,6 +7,7 @@ export const createItemSchema = z.object({
   categoryId: z.number().int().positive(),
   notes: z.string().optional(),
   productUrl: z.string().url().optional().or(z.literal("")),
+  imageFilename: z.string().optional(),
 });
 
 export const updateItemSchema = createItemSchema.partial().extend({
@@ -43,6 +44,7 @@ export const createCandidateSchema = z.object({
   categoryId: z.number().int().positive(),
   notes: z.string().optional(),
   productUrl: z.string().url().optional().or(z.literal("")),
+  imageFilename: z.string().optional(),
 });
 
 export const updateCandidateSchema = createCandidateSchema.partial();