feat(gitlab): Implement GitLab REST API client and type definitions

Provides a typed interface to the GitLab API with pagination support.

src/gitlab/types.rs - API response type definitions:
- GitLabIssue: Full issue payload with author, assignees, labels
- GitLabDiscussion: Discussion thread with notes array
- GitLabNote: Individual note with author, timestamps, body
- GitLabAuthor/GitLabUser: User information with avatar URLs
- GitLabProject: Project metadata from /api/v4/projects
- GitLabVersion: GitLab instance version from /api/v4/version
- GitLabNotePosition: Line-level position for diff notes
- All types derive Deserialize for JSON parsing

src/gitlab/client.rs - HTTP client with authentication:
- Bearer token authentication from config
- Base URL configuration for self-hosted instances
- Paginated iteration via keyset or offset pagination
- Automatic Link header parsing for next page URLs
- Per-page limit control (default 100)
- Methods: get_user(), get_version(), get_project()
- Async stream for issues: list_issues_paginated()
- Async stream for discussions: list_issue_discussions_paginated()
- Respects GitLab rate limiting via response headers

src/gitlab/transformers/ - API to database mapping:

transformers/issue.rs - Issue transformation:
- Maps GitLabIssue to IssueRow for database insert
- Extracts milestone ID and due date
- Normalizes author/assignee usernames
- Preserves label IDs for junction table
- Returns IssueWithMetadata including label/assignee lists

transformers/discussion.rs - Discussion transformation:
- Maps GitLabDiscussion to NormalizedDiscussion
- Extracts thread metadata (resolvable, resolved)
- Flattens notes to NormalizedNote with foreign keys
- Handles system notes vs user notes
- Preserves note position for diff discussions

transformers/mod.rs - Re-exports all transformer types

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/gitlab/mod.rs

@@ -0,0 +1,15 @@
+//! GitLab API client and types.
+
+pub mod client;
+pub mod transformers;
+pub mod types;
+
+pub use client::GitLabClient;
+pub use transformers::{
+    IssueRow, IssueWithMetadata, MilestoneRow, NormalizedDiscussion, NormalizedNote,
+    transform_discussion, transform_issue, transform_notes,
+};
+pub use types::{
+    GitLabAuthor, GitLabDiscussion, GitLabIssue, GitLabNote, GitLabNotePosition, GitLabProject,
+    GitLabUser, GitLabVersion,
+};