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>

config.py

@@ -1,19 +1,18 @@
-from dataclasses import dataclass, field
-import os
+from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
-@dataclass
-class APIConfig:
-    url: str = field(default_factory=lambda: os.getenv("YOULBOT_API_URL", "http://localhost:8000"))
-    token: str = field(default_factory=lambda: os.getenv("YOULBOT_API_TOKEN", ""))
-    timeout: int = 180
-
-
-@dataclass
-class AppConfig:
-    api: APIConfig = field(default_factory=APIConfig)
-    whisper_model_size: str = field(default_factory=lambda: os.getenv("WHISPER_MODEL_SIZE", "small"))
-    tts_voice: str = field(default_factory=lambda: os.getenv("TTS_VOICE", "Yuna"))
-    tts_edge_voice: str = field(default_factory=lambda: os.getenv("TTS_EDGE_VOICE", "ko-KR-SunHiNeural"))
+class AppConfig(BaseSettings):
+    # API (env: YOULBOT_API_URL, YOULBOT_API_TOKEN, YOULBOT_API_TIMEOUT)
+    youlbot_api_url: str = "http://localhost:8000"
+    youlbot_api_token: str = ""
+    youlbot_api_timeout: int = 180
+    # STT/TTS (env: WHISPER_MODEL_SIZE, TTS_VOICE, TTS_EDGE_VOICE)
+    whisper_model_size: str = "small"
+    tts_voice: str = "Yuna"
+    tts_edge_voice: str = "ko-KR-SunHiNeural"
+    # 서버 / 로깅
+    log_level: str = "INFO"
     server_host: str = "0.0.0.0"
     server_port: int = 7860
+
+    model_config = SettingsConfigDict(env_file=".env", extra="ignore")