feat(01-01): add database schema, shared Zod schemas, seed, and test infrastructure

- Create Drizzle schema with items, categories, and settings tables
- Set up database connection singleton with WAL mode and foreign keys
- Add seed script for default Uncategorized category
- Create shared Zod validation schemas for items and categories
- Export TypeScript types inferred from Zod and Drizzle schemas
- Add in-memory SQLite test helper for isolated test databases
- Wire seed into Hono server startup

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

src/db/index.ts

@@ -0,0 +1,9 @@
+import { Database } from "bun:sqlite";
+import { drizzle } from "drizzle-orm/bun-sqlite";
+import * as schema from "./schema.ts";
+
+const sqlite = new Database("gearbox.db");
+sqlite.run("PRAGMA journal_mode = WAL");
+sqlite.run("PRAGMA foreign_keys = ON");
+
+export const db = drizzle(sqlite, { schema });