chore: migrate Settings to Pydantic v2 ConfigDict

Replace deprecated Pydantic v1 `class Config:` inner class with
`model_config = SettingsConfigDict(...)` to eliminate the deprecation
warning emitted on every test run.

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

app/config/settings.py

@@ -1,5 +1,5 @@
 from typing import Literal
-from pydantic_settings import BaseSettings
+from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
 class Settings(BaseSettings):
@@ -54,9 +54,7 @@ class Settings(BaseSettings):
 
     ENV: Literal["dev", "prod"] = "dev"
 
-    class Config:
-        env_file = ".env"
-        env_file_encoding = "utf-8"
+    model_config = SettingsConfigDict(env_file=".env", env_file_encoding="utf-8")
 
 
 settings = Settings()