Summary

Introduces a unified GitLab REST API client, OAuth session storage backends, and fixes all ESLint errors.

Changes

Unified GitLab REST API Client

• Add src/utils/gitlab-api.ts - unified client for GitLab API calls
• Centralized URL building with base URL from config
• Query parameters serialization with undefined/null filtering
• Request body encoding (JSON or form-urlencoded)
• Automatic GID cleanup from responses
• Handle 204 No Content responses correctly
• Type-safe request/response handling

OAuth Session Storage Backends

New pluggable storage architecture for OAuth sessions:

• Memory Backend (default) - In-memory storage, sessions lost on restart
• File Backend - JSON file-based persistence with configurable save interval
• PostgreSQL Backend - Production-grade storage via Prisma ORM

Storage selection via environment:
```bash
OAUTH_STORAGE_TYPE=memory|file|postgresql
OAUTH_STORAGE_FILE_PATH=./data/oauth-sessions.json
OAUTH_STORAGE_POSTGRESQL_URL=postgresql://...
```

OAuth Flow Implementation (Dual Flow Support)

Authorization Code Flow (for Claude.ai Custom Connectors):

1. Client redirects to `/authorize` with `redirect_uri`
2. Server redirects to GitLab OAuth
3. GitLab redirects back to `/oauth/callback`
4. Server exchanges code for tokens
5. Server redirects to client with authorization code
6. Client exchanges code at `/token` endpoint

Device Flow (for CLI clients, Claude Desktop):

1. Client requests `/authorize` without `redirect_uri`
2. Server returns device code and verification URL
3. User visits GitLab, enters device code
4. Client polls `/oauth/poll` for completion
5. Server exchanges GitLab token for MCP session token

Flow auto-selection based on `redirect_uri` presence in authorization request.

New OAuth Endpoints

• `/oauth/callback` - GitLab OAuth callback handler
• `/oauth/poll` - Device flow polling endpoint
• `/register` - Dynamic client registration (RFC 7591)

Registry Refactoring

• Refactor labels, wiki, variables, milestones registries
• Use new gitlab-api utility for cleaner request handling
• Consistent pattern across all entity registries

Lint Fixes

• Fix unnecessary type assertion in gitlab-api.ts
• Fix Prisma type resolution with explicit interfaces (GenericPrismaClient pattern)
• Fix logger mocks in integration and unit tests
• All 101 errors resolved with proper typing, zero eslint-disable comments

Benefits

• Cleaner, more consistent API calls
• Type-safe request/response handling
• Centralized error handling
• Production-ready OAuth session persistence
• Support for both web and CLI OAuth flows
• Zero lint errors