fix(hf-daily-papers): default to yesterday + retry-with-fallback + earlier cron

[jeremymanning]

Root cause

The 2026-05-15T00:15Z scheduled run failed with HTTP 400. Analysis:

• Cron fires at 23:59 UTC, but GitHub Actions queues routinely delay 5-30 min.
• The actual run started at 00:15:10Z — 16 minutes past midnight UTC.
• _today_utc() returns the new UTC day (e.g. 2026-05-15) after midnight crossover.
• HF's daily-papers bucket for that day isn't populated until HF's late-afternoon editorial pipeline runs — https://huggingface.co/api/daily_papers?date=2026-05-15 (queried at 00:16 UTC) returned HTTP 400.
• The error propagates out as exit code 2 → workflow fails → no papers filed.

Three layered fixes

1. Default to yesterday-UTC

src/llmxive/hf_daily_papers.py _today_utc() now returns (now_utc - 1 day). HF buckets are filled late in the day; yesterday is the safest default.

2. Retry-with-fallback on 400/404

_fetch_daily_json(date, fallback_days=1) now:

• Tries date first.
• On HTTP 400 or 404 (bucket not populated), walks back one day and retries.
• On HTTP 5xx (transient HF issue), propagates unchanged — don't silently swallow real failures.
• Returns (effective_date, payload) so callers can log when the fallback fired.

fetch_top_papers() updated to return (effective_date, list[Paper]). submit_top_papers() unpacks and logs a fallback message to stderr when triggered.

3. Move cron from 23:59 UTC → 08:00 UTC

.github/workflows/hf-daily-papers.yml schedule: "59 23 * * *" → "0 8 * * *". At 08:00 UTC, yesterday's bucket is guaranteed-published regardless of any scheduler drift.

Tests

5 new TestDateFallback tests in tests/unit/test_hf_daily_papers.py:

• test_today_utc_defaults_to_yesterday — _today_utc returns (now - 1 day)
• test_fetch_falls_back_on_400 — 400 triggers exactly one fallback retry
• test_fetch_falls_back_on_404 — 404 also triggers fallback
• test_fetch_does_not_swallow_5xx — 503 propagates unchanged
• test_fetch_raises_when_all_fallback_attempts_400 — all-400 chain raises the last HTTPError

17/17 HF tests pass (existing 12 + 5 new).

Backward compat

The only API-shape change is fetch_top_papers returning a tuple. All 5 existing test callsites updated to unpack. No external callers exist outside this module + tests.

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