### Description
- Endpoint: GET `/me`, returns the whole user data.
### Package Changes
_No response_
### Screenshots
_No response_
### Reference
Resolves#100
### Checklist
- [x] A milestone is set
- [x] The related issuse has been linked to this branch
Reviewed-on: #102
Co-authored-by: SquidSpirit <squid@squidspirit.com>
Co-committed-by: SquidSpirit <squid@squidspirit.com>
### Description
This PR introduces the functionality to persist user information in the database. When a user logs in via OIDC for the first time, a new user record is created. Subsequent logins will retrieve the existing user data from the database.
This change ensures that users have a persistent identity within our system, identified by their unique combination of OIDC issuer and subject ID.
#### Key Changes
* **User Persistence Logic**:
* In `ExchangeAuthCodeUseCase`, after successfully exchanging the authorization code, the logic now checks if the user exists in our database using their `issuer` and `source_id`.
* If the user is not found (`AuthError::UserNotFound`), a new record is created in the `user` table.
* The `User` entity returned by the use case now contains the internal database `id`.
* **Database Integration in Auth Feature**:
* Introduced a new `UserDbService` trait and its `sqlx`-based implementation, `UserDbServiceImpl`, to handle database operations for users.
* The `AuthRepository` is extended to include methods for querying (`get_user_by_source_id`) and saving (`save_user`) users, delegating the calls to the new `UserDbService`.
* The dependency injection container in `server/src/container.rs` has been updated to provide the `UserDbServiceImpl` to the `AuthRepositoryImpl`.
* **Domain and Data Model Updates**:
* The `User` domain entity now includes `id` (the database primary key) and `issuer` (from OIDC claims) to uniquely identify a user across different identity providers.
* The `UserResponseDto` now exposes the internal `id` instead of the `source_id`.
* **Session Management**:
* The user's session now stores the database `user_id` (`i32`) instead of the entire user object. This is more efficient and secure.
* Session keys have been centralized into a `constants.rs` file for better maintainability.
#### Database Changes
* A new database migration has been added to create the `user` table.
* The table includes columns for `id`, `issuer`, `source_id`, `displayed_name`, and `email`.
* A **`UNIQUE` index** has been created on `(source_id, issuer)` to guarantee that each user from a specific identity provider is stored only once.
#### Refactoring
* Minor refactoring in the `image` feature to change `id: Option<i32>` to `id: i32` for consistency with the new `User` entity model.
### Package Changes
_No response_
### Screenshots
_No response_
### Reference
Resolves#94
### Checklist
- [x] A milestone is set
- [x] The related issuse has been linked to this branch
Reviewed-on: #96
Co-authored-by: SquidSpirit <squid@squidspirit.com>
Co-committed-by: SquidSpirit <squid@squidspirit.com>
