feat: tenant AI chat agent with function calling

Actionable AI assistant that uses per-tenant Azure OpenAI credentials
to execute natural language commands against the render pipeline.

Backend:
- ChatMessage model + migration (session-based conversations)
- Chat service with 10 OpenAI function-calling tools:
  list_orders, search_products, create_order, dispatch_renders,
  get_order_status, set_material_override, set_render_overrides,
  get_render_stats, check_materials, query_database
- All tools tenant-scoped (queries filtered by tenant_id)
- Write operations use httpx to call backend API internally
- Chat API: POST /chat/messages, GET /chat/sessions, DELETE session
- Conversation history preserved in DB (last 50 messages per session)

Frontend:
- Slide-out ChatPanel (right side, w-96, animated)
- User/assistant message styling with avatars and timestamps
- Session management (new chat, session history, delete)
- Typing indicator while waiting for AI response
- Floating chat button in bottom-right corner
- Error state for unconfigured AI tenants

Example: "Render all Kugellager products as WebP at 1024x1024"
→ Agent calls search_products + create_order + dispatch_renders

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/main.py

@@ -26,6 +26,7 @@ from app.domains.media.router import router as media_router
 from app.api.routers.asset_libraries import router as asset_libraries_router
 from app.domains.admin.dashboard_router import router as dashboard_router
 from app.api.routers.task_logs import router as task_logs_router
+from app.api.routers.chat import router as chat_router
 
 
 @asynccontextmanager
@@ -95,6 +96,7 @@ app.include_router(asset_libraries_router, prefix="/api")
 app.include_router(dashboard_router, prefix="/api")
 app.include_router(task_logs_router, prefix="/api")
 app.include_router(global_render_positions_router, prefix="/api")
+app.include_router(chat_router, prefix="/api")
 
 
 @app.get("/health")