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>

tests/conftest.py

@@ -19,6 +19,25 @@ from cartsnitch_api.database import get_db
 from cartsnitch_api.main import create_app
 from cartsnitch_api.models import Base
 
+TEST_JWT_SECRET = secrets.token_urlsafe(32)
+TEST_SERVICE_KEY = secrets.token_urlsafe(32)
+TEST_FERNET_KEY = "7reF42nmTwbdN21PBoubGp7h_FU8qSimstmlaMLoRK8="
+
+
+@pytest.fixture(autouse=True)
+def setup_test_settings():
+    original_jwt = cartsnitch_settings.jwt_secret_key
+    original_service = cartsnitch_settings.service_key
+    original_fernet = cartsnitch_settings.fernet_key
+    cartsnitch_settings.jwt_secret_key = TEST_JWT_SECRET
+    cartsnitch_settings.service_key = TEST_SERVICE_KEY
+    cartsnitch_settings.fernet_key = TEST_FERNET_KEY
+    yield
+    cartsnitch_settings.jwt_secret_key = original_jwt
+    cartsnitch_settings.service_key = original_service
+    cartsnitch_settings.fernet_key = original_fernet
+
+
 TEST_DATABASE_URL = "sqlite+aiosqlite:///:memory:"
 
 
@@ -60,7 +79,8 @@ async def db_engine():
     async with engine.begin() as conn:
         await conn.run_sync(Base.metadata.create_all)
         # Create Better-Auth tables (not managed by SQLAlchemy models)
-        await conn.execute(text("""
+        await conn.execute(
+            text("""
             CREATE TABLE IF NOT EXISTS sessions (
                 id TEXT PRIMARY KEY,
                 token TEXT NOT NULL UNIQUE,
@@ -71,8 +91,10 @@ async def db_engine():
                 created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL,
                 updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL
             )
-        """))
-        await conn.execute(text("""
+        """)
+        )
+        await conn.execute(
+            text("""
             CREATE TABLE IF NOT EXISTS accounts (
                 id TEXT PRIMARY KEY,
                 user_id TEXT NOT NULL,
@@ -88,8 +110,10 @@ async def db_engine():
                 created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL,
                 updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL
             )
-        """))
-        await conn.execute(text("""
+        """)
+        )
+        await conn.execute(
+            text("""
             CREATE TABLE IF NOT EXISTS verifications (
                 id TEXT PRIMARY KEY,
                 identifier TEXT NOT NULL,
@@ -98,7 +122,8 @@ async def db_engine():
                 created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL,
                 updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL
             )
-        """))
+        """)
+        )
 
     yield engine
 
@@ -133,7 +158,9 @@ async def client(db_engine):
     app.dependency_overrides.clear()
 
 
-async def _create_test_user_and_session(client: AsyncClient, db_engine, **user_overrides) -> tuple[dict, str]:
+async def _create_test_user_and_session(
+    client: AsyncClient, db_engine, **user_overrides
+) -> tuple[dict, str]:
     """Create a test user and a valid session directly in the DB.
 
     Returns (user_dict, session_token).  Better-Auth stores the raw token