Add shared ai-service container as AI provider intermediary

All feature containers now POST messages to ai-service (port 8010) instead
of calling AI providers directly. ai-service routes to LM Studio, Ollama,
or Anthropic based on /config/ai_service_config.json. doc-service AI
providers removed; replaced by httpx ai_client.py. Backend settings
restructured to /api/settings/ai. Frontend gets dedicated AIAdminSettingsPage
and AI Service card in AppsPage.

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

backend/app/core/config.py

@@ -15,6 +15,8 @@ class Settings(BaseSettings):
 
     CORS_ORIGINS: list[str] = ["http://localhost:5173"]
 
+    AI_SERVICE_URL: str = "http://ai-service:8010"
+
     @field_validator("JWT_PRIVATE_KEY", "JWT_PUBLIC_KEY", mode="before")
     @classmethod
     def expand_newlines(cls, v: str) -> str: