docs: mark ai-assistant service as deprecated, not in live path

The production frontend calls backend-rust/apps/users/src/handlers/ai.rs
directly. This service is an orphaned parallel build. Added a prominent
banner in main.rs pointing devs to the correct extension points.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main.rs

@@ -1,3 +1,20 @@
+// ╔══════════════════════════════════════════════════════════════════════════╗
+// ║  DEPRECATED — NOT IN THE LIVE REQUEST PATH                              ║
+// ║                                                                          ║
+// ║  This service was built in parallel with the AI stack inside             ║
+// ║  nxtgauge-backend-rust/apps/users/src/handlers/ai.rs.                   ║
+// ║  The production frontend (AiChatWidget.tsx) calls the backend-rust       ║
+// ║  stack directly via /api/ai/*. This service is never called in           ║
+// ║  production.                                                             ║
+// ║                                                                          ║
+// ║  Do NOT add new AI features here. Extend backend-rust instead:          ║
+// ║    • Chat / intent:  handlers/ai.rs → ask_ash / ai_router               ║
+// ║    • Confirm flow:   handlers/ai.rs → ai_chat_confirm                   ║
+// ║    • Credits/plans:  handlers/ai_credits.rs                             ║
+// ║    • Generation:     handlers/ai.rs (generate-cover-letter,             ║
+// ║                        generate-job-field, tailor-resume, etc.)         ║
+// ╚══════════════════════════════════════════════════════════════════════════╝
+
 mod actions;
 mod auth;
 mod chat;