fix: remove hardcoded default secrets from API config

Remove dangerous default values for jwt_secret_key, service_key, and
fernet_key. Add startup validation that raises RuntimeError if these
secrets are not set via environment variables or contain placeholder
values.

Add test fixture to provide explicit test values for these secrets,
ensuring existing tests continue to pass.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/cartsnitch_api/config.py

@@ -13,14 +13,13 @@ class Settings(BaseSettings):
     )
     redis_url: str = "redis://localhost:6379/0"
 
-    jwt_secret_key: str = "change-me-in-production"
+    jwt_secret_key: str
     jwt_algorithm: str = "HS256"
     jwt_access_token_expire_minutes: int = 15
     jwt_refresh_token_expire_days: int = 7
 
-    service_key: str = "change-me-in-production"
-    # Valid Fernet key for local dev — MUST be overridden in production
-    fernet_key: str = "7reF42nmTwbdN21PBoubGp7h_FU8qSimstmlaMLoRK8="
+    service_key: str
+    fernet_key: str
 
     auth_service_url: str = "http://auth:3001"
 
@@ -35,9 +34,26 @@ class Settings(BaseSettings):
     rate_limit_window_seconds: int = 60
     rate_limit_enabled: bool = True
 
+    _PLACEHOLDER_VALUES = {"change-me-in-production"}
+
     @model_validator(mode="after")
-    def validate_fernet_key(self):
-        """Validate fernet_key is a valid 32-byte url-safe base64 key at startup."""
+    def validate_secrets(self):
+        if not self.jwt_secret_key or self.jwt_secret_key in self._PLACEHOLDER_VALUES:
+            raise ValueError(
+                "CARTSNITCH_JWT_SECRET_KEY must be set to a secure value. "
+                'Generate one with: python -c "import secrets; print(secrets.token_urlsafe(32))"'
+            )
+        if not self.service_key or self.service_key in self._PLACEHOLDER_VALUES:
+            raise ValueError(
+                "CARTSNITCH_SERVICE_KEY must be set to a secure value. "
+                'Generate one with: python -c "import secrets; print(secrets.token_urlsafe(32))"'
+            )
+        if not self.fernet_key or self.fernet_key in self._PLACEHOLDER_VALUES:
+            raise ValueError(
+                "CARTSNITCH_FERNET_KEY must be set to a valid Fernet key. "
+                "Generate one with: python -c "
+                "'from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())'"
+            )
         try:
             decoded = base64.urlsafe_b64decode(self.fernet_key.encode())
             if len(decoded) != 32: