feat: add web UI with login, CRUD, admin, and API key management

- Add login page with JWT authentication
- Add dashboard with stats and quick actions
- Add links management page (full CRUD with search)
- Add collections management page
- Add API key management page with copy-to-clipboard
- Add admin user management page (admin only)
- Fix UUID type mismatches across all endpoints
- Add updated_at column to api_keys and audit_log in schema.sql
- Fix DB_PASSWORD default in docker-compose.yml
- Add PyJWT to requirements.txt
- Fix API docs URL (/docs instead of /api/docs)
- Improve JS error handling (show actual messages)
- Rewrite conftest.py with proper DB lifecycle management
- Add 42 new integration tests (84 total, all passing)
  - test_admin.py: 15 tests for admin endpoints
  - test_auth_extended.py: 9 tests for API key CRUD
  - test_tags.py: 12 tests for tag endpoints
  - test_sync.py: 6 tests for sync endpoints

LinkSyncServer/docker-compose.yml

@@ -6,7 +6,7 @@ services:
     ports:
       - "5000:5000"
     environment:
-      - DATABASE_URL=postgresql://linksync:${DB_PASSWORD}@db:5432/linksync
+      - DATABASE_URL=postgresql://linksync:${DB_PASSWORD:-password}@db:5432/linksync
       - SECRET_KEY=${SECRET_KEY:-$(openssl rand -base64 32)}
       - ADMIN_USERNAME=${ADMIN_USERNAME:-admin}
       - ADMIN_PASSWORD=${ADMIN_PASSWORD:-admin123}