Phase 28: P3 — Pydantic Settings, dependency-injector IoC, tenacity retry

- config.py: dataclasses → pydantic-settings BaseSettings (flat AppConfig,
  env vars auto-loaded from .env, type-safe validation)
- api_client.py: HTTPAPIClient takes AppConfig directly (APIConfig removed);
  tenacity retry on 5 methods (reset/ingest/list/delete/feedback) —
  retries on 5xx + TransportError, 3 attempts, exponential backoff 1-8s
- container.py: manual DI → dependency_injector DeclarativeContainer with
  providers.Singleton; Container() needs no args
- app.py: container.X → container.X() calls, remove AppConfig import
- requirements.txt: add pydantic-settings, tenacity, dependency-injector

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

container.py

@@ -1,41 +1,15 @@
-"""수동 DI 컨테이너."""
+from dependency_injector import containers, providers
+
 from api_client import HTTPAPIClient
 from config import AppConfig
 from services import ChatService, DocumentService, TTSService
 
 
-class Container:
-    def __init__(self, config: AppConfig):
-        self._config = config
-        self._api_client: HTTPAPIClient | None = None
-        self._chat_service: ChatService | None = None
-        self._document_service: DocumentService | None = None
-        self._tts_service: TTSService | None = None
+class Container(containers.DeclarativeContainer):
+    config = providers.Singleton(AppConfig)
 
-    @property
-    def config(self) -> AppConfig:
-        return self._config
+    api_client = providers.Singleton(HTTPAPIClient, config=config)
 
-    @property
-    def api_client(self) -> HTTPAPIClient:
-        if self._api_client is None:
-            self._api_client = HTTPAPIClient(self._config.api)
-        return self._api_client
-
-    @property
-    def chat_service(self) -> ChatService:
-        if self._chat_service is None:
-            self._chat_service = ChatService(self.api_client)
-        return self._chat_service
-
-    @property
-    def document_service(self) -> DocumentService:
-        if self._document_service is None:
-            self._document_service = DocumentService(self.api_client)
-        return self._document_service
-
-    @property
-    def tts_service(self) -> TTSService:
-        if self._tts_service is None:
-            self._tts_service = TTSService(self._config)
-        return self._tts_service
+    chat_service = providers.Singleton(ChatService, api_client=api_client)
+    document_service = providers.Singleton(DocumentService, api_client=api_client)
+    tts_service = providers.Singleton(TTSService, config=config)