[Connector] GitLab Group Connector: Mass Indexing and Integration Support

[Anthony-Bible]

Business Requirements

• Allow administrators to connect the codechunking platform to GitLab, supporting both gitlab.com and self-hosted GitLab instances.
• Users must be able to authenticate securely (preferably via OAuth 2.0) to grant access to one or more GitLab groups.
• Administrators can select groups and optionally include subgroups/descendants for recursive indexing.
• Support repository filtering by name pattern, activity date, and visibility (public/private/internal).
• API endpoint(s) must allow users to trigger mass indexing jobs for all or filtered subsets of repositories within a group.
• Connector setup should support storing and revoking OAuth tokens securely, and refreshing tokens as needed.
• Maintain an audit trail of connections, indexing jobs, and repository discovery outcomes for traceability.
• Provide clear API responses and error handling for all connector operations (connection, listing, indexing, revocation, etc.).

Technical Requirements

• Implement OAuth 2.0 authentication flow for GitLab as a first-class integration option. Store tokens securely (encrypted at rest, never log sensitive data).
• Design connector configuration to support both gitlab.com and arbitrary self-hosted GitLab URLs. Allow API base URL to be configurable.
• Fetch group/project listing via GitLab's REST API, supporting pagination and deep-nested subgroup traversal.
• Implement repository filtering logic server-side—accept filter params (name, visibility, last activity, etc.) and apply to the discovery process.
• Implement connector lifecycle endpoints: connect (initiate OAuth), list (view connected groups), index (trigger mass indexing), disconnect (revoke).
• Store metadata on discovered repositories (group IDs, project IDs, last indexed timestamp, etc.) for tracking and deduplication.
• Robust error handling: propagate GitLab and system errors to users with actionable messages.
• Provide support for automated re-indexing on a schedule or via webhook/event (future work: outline event-driven discovery triggers).
• Write unit/integration tests covering auth, group discovery, filtering, and mass indexing invocation.

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This sub-issue tracks the implementation of a GitLab connector as part of #35. See parent for overall context and next steps.