fix(letters): add missing error logging to /api/letters/{id}/extract

[aircode610]

Summary

• Closes a remaining gap from 500 error on /api/letters upload when PDF has no text layer #8: the /api/letters/{id}/extract endpoint's generic except Exception handler silently swallowed errors — the comment misleadingly claimed they would be "Logged by unhandled_exception_handler", but KlarHTTPException extends HTTPException which is caught by klar_exception_handler, so the original traceback was silently lost.
• Added logging import + logger = logging.getLogger("klar.letters") to letters.py
• Added logger.exception() to the generic except Exception handler and logger.info() to the typed PdfRenderError/ExtractionError handlers — matching what public.py already does
• Added 3 new regression tests covering the /api/letters/{id}/extract path (scanned PDF → EXTRACTION_FAILED, corrupt PDF → PDF_RENDER_FAILED, unexpected error → generic 502 without leaking internals)

Context

PR #13 fixed the core issue #8 crash across all three entry points (POST /letters, SSE /process, sync /extract). The typed error handling was correctly applied everywhere, but logging was only added to public.py and orchestrator.py — the letters.py handler was left with a silent except Exception that makes production debugging impossible on the /api/letters/{id}/extract route.

Test plan

• 24/24 regression tests pass (backend/tests/test_scanned_pdf_graceful.py)
• ruff clean on both modified files
• New tests cover: ExtractionError → 502 EXTRACTION_FAILED, PdfRenderError → 502 PDF_RENDER_FAILED, generic RuntimeError → 502 without leaking secrets

🤖 Generated with Claude Code

[aircode610]

🔍 TODO / FIXME Scan

I checked every added and modified line in this PR for TODO, FIXME, HACK, and XXX comments.

Result: ✅ No TODO or FIXME comments found.

The PR is clean — no deferred-work markers were introduced in either backend/app/routers/letters.py or backend/tests/test_scanned_pdf_graceful.py.