♻️ Require explicit SECRET_KEY (#2425)

## AI Disclaimer

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

backend/app/core/config.py

@@ -1,4 +1,3 @@
-import secrets
 import warnings
 from typing import Annotated, Any, Literal, Self
 
@@ -30,7 +29,7 @@ class Settings(BaseSettings):
         extra="ignore",
     )
     API_V1_STR: str = "/api/v1"
-    SECRET_KEY: str = secrets.token_urlsafe(32)
+    SECRET_KEY: str
     # 60 minutes * 24 hours * 8 days = 8 days
     ACCESS_TOKEN_EXPIRE_MINUTES: int = 60 * 24 * 8
     FRONTEND_HOST: str = "http://localhost:5173"