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/utils/cors.ts

@@ -0,0 +1,21 @@
+export const corsHeaders = {
+  'Access-Control-Allow-Origin': '*',
+  'Access-Control-Allow-Methods': 'GET, POST, PUT, DELETE, OPTIONS',
+  'Access-Control-Allow-Headers': 'Content-Type'
+}
+
+export function optionsResponse() {
+  return new Response(null, { headers: corsHeaders })
+}
+
+export function jsonResponse(data: unknown, status = 200) {
+  return Response.json(data, { status, headers: corsHeaders })
+}
+
+export function errorResponse(error: string, status = 400) {
+  return Response.json({ error }, { status, headers: corsHeaders })
+}
+
+export function notFoundResponse() {
+  return new Response('Not Found', { status: 404, headers: corsHeaders })
+}