Add text search by student name and subject to admin requests

[Taleef7]

Context

docs/plan-CorvEd.md lists admin request text search as still TODO. The current request filters support status/subject/level, but not free-text search by student/requester name or subject.

Acceptance criteria

• /admin/requests supports a search query for student/requester names and/or subject text.
• Search composes with existing status, subject, and level filters.
• The UI exposes a clear search input and preserves query parameters predictably.
• Empty and no-result states are handled.

References

• docs/plan-CorvEd.md section 4.3