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/providers/base.py

@@ -0,0 +1,23 @@
+from abc import ABC, abstractmethod
+
+from app.schemas.chat import ChatMessage
+
+
+class AIProvider(ABC):
+    provider_name: str = "unknown"
+    model_name: str = "unknown"
+
+    @abstractmethod
+    async def chat(
+        self,
+        messages: list[ChatMessage],
+        max_tokens: int,
+        temperature: float,
+    ) -> tuple[str, int | None, int | None]:
+        """
+        Send messages to the provider and return (content, input_tokens, output_tokens).
+        Raises:
+            ProviderConnectionError: on network / auth failure
+            ProviderTimeoutError: on request timeout
+        """
+        ...