feat: conservatively adapt selected PR #29 improvements

[powerycy]

What changed

• Add configuration-page upload support for text-layer PDF resumes and convert extracted text to Markdown while retaining the existing MD and DOCX paths.
• Return explicit errors for encrypted, damaged, scanned, or otherwise textless PDFs; add the required pypdf runtime dependency.
• Normalize DeepSeek model IDs to lowercase only in the outbound OpenAI-compatible request. Other providers and custom compatible endpoints preserve their configured model IDs.
• Keep approved jobs without a greeting visible and recoverable using the current data model, including deleted_at filtering.
• Fix the confirmation handoff race that was reproduced when the button is clicked before confirmation_event is attached. The selected IDs are queued and consumed when the event is created.
• Update the checked-in frontend build served by the backend.
• Credit @Nourishman in the contribution leaderboard with the exact adapted scope.

Attribution

These changes reimplement selected low-risk improvements inspired by Nourishman's PR #29. The old PR was not merged or cherry-picked wholesale.

No Co-authored-by trailer is used because the public commit metadata does not reliably associate the exposed commit email with the GitHub contributor identity.

Intentionally excluded

• Windows start/stop scripts and the standalone Chrome launcher.
• Send-window, force, and DEADLINE_MODES behavior changes.
• The old README rewrite, old build output, and any version downgrade.
• General cancellation/stop logic already implemented more completely on current main.

Validation

• Related backend suite: 113 passed.
• Full backend suite: 302 passed, 11 subtests passed.
• Frontend TypeScript and Vite production build passed; checked-in dist was regenerated.
• CLI help/version/import smoke checks passed on version 2.3.0.
• git diff --check passed.
• Targeted Ruff check for the new PDF upload module passed. The repository-wide advisory Ruff run still reports the existing baseline findings; CI marks that step continue-on-error.
• No bosshunter run, collection, scoring, greeting generation, monitoring, or sending command was executed.

Contribution percentage basis

Nourishman is conservatively assigned 8% for the three adapted low-risk areas above. Excluded portions of PR #29 are not counted. Existing accepted contributors are retained and proportionally re-normalized with integer rounding so the leaderboard remains at 100%.