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/doc-service/app/core/config.py

@@ -6,6 +6,7 @@ class Settings(BaseSettings):
     DATABASE_URL: str = "postgresql+asyncpg://postgres:password@db:5432/destroying_sap"
     DATA_DIR: str = "/data/documents"
     CONFIG_PATH: str = "/config/doc_service_config.json"
+    AI_SERVICE_URL: str = "http://ai-service:8010"
 
     class Config:
         env_file = ".env"