feat: database layer - PostgreSQL schema + memory fallback

- drizzle-orm + postgres dependencies
- Full schema: 12 tables covering all modules
- Graceful fallback: no DATABASE_URL → memory mode
- drizzle-kit config for migrations
- Memory store as generic CRUD layer
- dev.ts auto-initializes DB on startup

package.json

@@ -10,13 +10,17 @@
     "build": "tsc && vite build",
     "lint": "eslint src/ --ext .ts,.tsx",
     "test": "vitest run",
-    "typecheck": "tsc --noEmit"
+    "typecheck": "tsc --noEmit",
+    "db:generate": "drizzle-kit generate",
+    "db:migrate": "drizzle-kit migrate"
   },
   "dependencies": {
     "@arco-design/web-react": "^2.66.0",
     "@hono/node-server": "^1.19.13",
     "@larksuiteoapi/node-sdk": "^1.60.0",
+    "drizzle-orm": "^0.45.2",
     "hono": "^4.7.0",
+    "postgres": "^3.4.9",
     "react": "^18.3.0",
     "react-dom": "^18.3.0"
   },