fix: backend and UI bugs

[rafaelsilva29]

Description

Bug fixes across the Explorer UI and auth server tracer repository found during post-merge review.

Backend

• tracer_repository.py: Use .is_(None) instead of == None for SQLAlchemy null checks
• tracer_repository.py: Add implicit event_type == MCPCallStarted filter when q is active — without it, other event types with a null tool field were silently excluded, skewing pagination totals
• tracer_repository.py: Replace filter_by(user_input_id=str) with .where(Trace.user_input_id == UUID(...)) to avoid type mismatch on PostgreSQL UUID columns

Frontend

• use-mas.ts / session-trace-sheet.tsx / mas-detail-page.tsx: Update useMASById/useMASApps to accept string | undefined; callers were passing id || '' which registered empty-string cache keys and fired queries with blank IDs
• auth-requests-page.tsx: Guard parseInt with Number.isFinite to prevent NaN page param from breaking all pagination controls; derive totalPages from server-returned page_size instead of local state
• auth-requests-table.tsx: Add !isFetching to the empty-state guard and an opacity overlay during refetches
• global-search.tsx: Add a clear (×) button that flushes debouncedQuery immediately; pass eventType: MCPCallStarted to the traces query for server-side pre-filtering
• data-table.tsx: Fix manualPagination to be controlled only by isServerPaginated; add pageCount for correct server-driven page navigation; add inline clear button to the search input

Type of Change

• Bugfix

Checklist

• I have read the contributing guidelines
• I have verified this change is not present in other open pull requests
• All code style checks pass