Add GitLab as a supported git provider for codebase linking

[lg-epitech]

Follow up to #22.

Extend the codebase linking architecture to support GitLab in addition to GitHub.

Scope:

• Add a provider abstraction that can support multiple git providers cleanly.
• Add GitLab-specific account linking and repository authorization flow.
• Persist linked GitLab repositories in the same codebase model with provider-specific connection metadata.
• Ensure workspace scoping, admin-only linking and unlinking, and secure server-side token handling remain consistent across providers.
• Keep the design compatible with future provider additions.

This should build on the architecture introduced for GitHub rather than duplicating provider-specific logic throughout the stack.

[linear-code]

DIS-20