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>

features/ai-service/app/core/config.py

@@ -0,0 +1,12 @@
+from pydantic_settings import BaseSettings
+
+
+class Settings(BaseSettings):
+    PROJECT_NAME: str = "ai-service"
+    CONFIG_PATH: str = "/config/ai_service_config.json"
+
+    class Config:
+        env_file = ".env"
+
+
+settings = Settings()