feat(backend): add pagination and search to admin teams endpoints

BREAKING CHANGE: Admin teams list endpoint response structure changed from flat array to paginated format

- Add pagination support to GET /api/admin/teams endpoint
  - Query params: limit (1-100, default 20), offset (default 0)
  - Response now includes data.teams and data.pagination metadata
  - Pagination metadata: total, limit, offset, has_more
- Create new GET /api/admin/teams/search endpoint
  - Filter by team name (partial, case-insensitive)
  - Includes same pagination support as list endpoint
- Add pagination schemas and validation helper to admin/teams/schemas.ts
- Register search route in admin teams index
- Update API specs (api-spec.json, api-spec.yaml)

Author: Lasim