Make admin_password optional so worker can start without it

backend/app/core/config.py

@@ -15,9 +15,10 @@ class Settings(BaseSettings):
     algorithm: str = "HS256"
     access_token_expire_minutes: int = 60 * 24 * 7  # 7 days
 
-    # Admin account
+    # Admin account - optional so the worker (which doesn't seed users) can start
+    # without it. The backend service checks this at seed time.
     admin_username: str = Field("admin", env="ADMIN_USERNAME")
-    admin_password: str = Field(..., env="ADMIN_PASSWORD")
+    admin_password: Optional[str] = Field(None, env="ADMIN_PASSWORD")
 
     # PocketID OIDC (optional)
     pocketid_issuer: Optional[str] = Field(None, env="POCKETID_ISSUER")
@@ -35,4 +36,4 @@ class Settings(BaseSettings):
         case_sensitive = False
 
 
-settings = Settings()
+settings = Settings()