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

drizzle.config.ts

@@ -0,0 +1,10 @@
+import { defineConfig } from 'drizzle-kit';
+
+export default defineConfig({
+  schema: './src/db/schema.ts',
+  out: './drizzle',
+  dialect: 'postgresql',
+  dbCredentials: {
+    url: process.env.DATABASE_URL || 'postgresql://flowpilot:flowpilot@localhost:5432/flowpilot',
+  },
+});