refactor: Modularize server into separate concerns

Split monolithic index.ts (~1400 lines) into modular structure:
- config.ts: Server configuration and constants
- db/: Database initialization, migrations, and seeds
- routes/: API handlers by domain (themes, canvas, components, etc.)
- services/: Terminal WebSocket server
- utils/: CORS helpers

Entry point now only coordinates initialization.

server/db/index.ts

@@ -0,0 +1,16 @@
+import { Database } from 'bun:sqlite'
+import { DB_PATH } from '../config'
+import { runMigrations } from './migrations'
+import { runSeeds } from './seeds'
+
+// Create database instance
+export const db = new Database(DB_PATH)
+
+// Initialize database
+export function initDatabase() {
+  runMigrations(db)
+  runSeeds(db)
+  console.log('[DB] SQLite initialized:', DB_PATH)
+}
+
+export { Database }