♻️ Use FASTAPI_ENV for development (#2427)

## AI Disclaimer

Using Codex with gpt-5.6-sol. Reviewed manually.

backend/app/core/config.py

@@ -33,7 +33,7 @@ class Settings(BaseSettings):
     # 60 minutes * 24 hours * 8 days = 8 days
     ACCESS_TOKEN_EXPIRE_MINUTES: int = 60 * 24 * 8
     FRONTEND_HOST: str = "http://localhost:5173"
-    ENVIRONMENT: Literal["local"] | None = None
+    FASTAPI_ENV: Literal["development"] | None = None
 
     BACKEND_CORS_ORIGINS: Annotated[
         list[AnyUrl] | str, BeforeValidator(parse_cors)
@@ -98,7 +98,7 @@ class Settings(BaseSettings):
                 f'The value of {var_name} is "changethis", '
                 "for security, please change it, at least for deployments."
             )
-            if self.ENVIRONMENT == "local":
+            if self.FASTAPI_ENV == "development":
                 warnings.warn(message, stacklevel=1)
             else:
                 raise ValueError(message)