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/seed.ts

@@ -0,0 +1,14 @@
+import { db } from "./index.ts";
+import { categories } from "./schema.ts";
+
+export function seedDefaults() {
+  const existing = db.select().from(categories).all();
+  if (existing.length === 0) {
+    db.insert(categories)
+      .values({
+        name: "Uncategorized",
+        emoji: "\u{1F4E6}",
+      })
+      .run();
+  }
+}