Add unit tests for models and services

- Implemented unit tests for the Package model, covering methods such as generateId, findById, findByName, and version management.
- Created unit tests for the Repository model, including repository creation, name validation, and retrieval methods.
- Added tests for the Session model, focusing on session creation, validation, and invalidation.
- Developed unit tests for the User model, ensuring user creation, password hashing, and retrieval methods function correctly.
- Implemented AuthService tests, validating login, token refresh, and session management.
- Added TokenService tests, covering token creation, validation, and revocation processes.

test/fixtures/cargo/demo-crate/README.md

@@ -0,0 +1,13 @@
+# demo-crate
+
+Demo crate for Stack.Gallery Registry e2e tests.
+
+## Usage
+
+```rust
+use demo_crate::greet;
+
+fn main() {
+    println!("{}", greet("World")); // Hello, World!
+}
+```