fix: harden uploaded audio polling and accessibility

[1074016045]

Summary

This PR delivers the first focused v0.6 increment for Uploaded Audio reliability, accessibility, and deterministic component-level polling coverage.

Added

• 32 focused React component tests for UploadedAudioPanel
• deterministic fake-timer and controlled-promise polling tests
• loading, empty, uploaded, queued, running, retrying, completed, failed, cancelled, and deleting state coverage
• explicit Transcribe and Delete action coverage
• polling visibility, abort, cleanup, recovery, and terminal-state coverage
• accessibility regression coverage for labels, busy states, alerts, status announcements, keyboard access, and focus preservation

Correctness fixes

• prevents stale in-flight polling responses from restoring an asset after successful deletion
• prevents callback identity changes from restarting the loading and polling lifecycle
• prevents terminal completion-callback failures from scheduling another polling timer
• rejects late responses from aborted or superseded requests
• clears recovered polling errors without erasing valid asset state
• invalidates and aborts polling before authoritative post-delete reload

Accessibility improvements

• announced initial loading state
• explicit cancelling/deleting status
• accurate per-asset aria-busy
• clearer accessible name for an in-progress destructive action
• request failures use alert semantics
• polling refreshes do not steal focus

This PR does not claim formal WCAG compliance.

Verification

• focused component tests: 32/32 passed, three consecutive runs
• Vitest: 29 files, 313/313 tests passed
• ESLint passed
• TypeScript typecheck passed
• Question Boundary evaluation passed with 100% accuracy/F1
• Question Understanding evaluation passed
• Playwright: 6/6 passed
• production build passed
• git diff --check passed
• no React warnings or unhandled promise rejections

Scope integrity

• pnpm-lock.yaml unchanged
• migrations 0000–0006 unchanged
• no 0007 migration created
• no database schema or API contract change
• no provider or dependency change

Deferred v0.6 work

• database backup, validation, and restore
• configurable retention
• content-free operational observability