fix(accent): degrade gracefully when OJAD is unavailable

[torrid-fish]

Problem

OJAD (www.gavo.t.u-tokyo.ac.jp) is currently globally unreachable (80/443 TCP connect timing out, ping 100% loss — confirmed from an independent external egress, not an IP block). Because every accent chunk POSTs to OJAD for the per-mora pitch contour, an OJAD outage made /MarkAccent/:

1. Hang for the full global 10s timeout (main.py httpx.AsyncClient(timeout=10.0)).
2. Re-raise the httpx.ConnectTimeout from get_ojad_result (except Exception: raise).
3. Surface as HTTP 500 via the pipeline's broad catch.

So users waited ~10s only to get a 500. Prod logs showed ~1898 ConnectTimeout / 1387 [OJAD] Request Failed in 24h.

Fix

Pitch accent from OJAD only enriches the furigana result, so an OJAD outage should degrade — not fail — the request.

• api/accent/ojad.py — Add a short per-request timeout OJAD_TIMEOUT = httpx.Timeout(5.0, connect=2.0) so a down OJAD fails fast (~2s) instead of hanging on the global 10s. Replace except Exception: raise with except httpx.HTTPError (covers connect/read timeouts, connection errors, and raise_for_status()'s status errors) → log a warning and raise a dedicated OJADUnavailableError.
• api/accent/pipeline.py — Catch OJADUnavailableError, set ojad_results = [] plus a warning, and continue. align_accent already emits accent_marking_type=0 for an empty OJAD list (its "MATCH FAILED" branch), so this yields furigana-only output at status 200. The broad except → 500 stays as a last-resort guard.
• api/accent/models.py — Add an optional, backward-compatible warning field to AccentResponse for degraded results.

Verification

• uv run ruff check api/accent/ and uv run mypy api/accent/ — both clean.
• OJAD-down simulation (mocked ConnectTimeout): get_ojad_result raises OJADUnavailableError, the short timeout is passed through, and the pipeline returns status=200, warning set, all accent_marking_type==0, furigana preserved. No 500, no 10s hang.
• Happy path (OJAD reachable): status=200, warning=null, real pitch contour preserved — no regression.

Close #59

[github-actions]

🛡️ PR Quality Check Summary

✅ PR Title: Passed (Length: 56/75, Format: OK). fix(accent): degrade gracefully when OJAD is unavailable
✅ Branch Name: Follows naming convention (fix/ojad-graceful-degradation)
✅ Commit Messages: All 1 commit(s) passed (Length, Format, Case)
✅ Conflicts: No merge conflict markers found
✅ Python Quality: All checks passed.

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