feat: finalize local OpenAI key flow and MCP docs

- add server-side .env.local OpenAI key management for the open-source app
- route AI features through the preferred local key path and lazy-load embed recovery
- rewrite README and docs for current MCP setup, schema, and fully-local guidance

Generated with Claude Code

instrumentation.ts

@@ -1,7 +1,6 @@
-import { startAutoEmbedRecovery } from '@/services/embedding/autoEmbedQueue';
-
 export async function register() {
   if (process.env.NEXT_RUNTIME === 'nodejs') {
+    const { startAutoEmbedRecovery } = await import('@/services/embedding/autoEmbedQueue');
     startAutoEmbedRecovery();
   }
 }