[static] RSS builder silently serves stale feeds and can overwrite good cache with an empty feed

[tiankaima]

Problem

Two freshness gaps in get_and_clean_feed (src/rss.py:18-61):

1. Silent staleness: feedparser.parse(url) never raises on HTTP/network errors — it returns a "bozo" feed with no entries. The function then hits if not feed.entries: return (src/rss.py:21-22) and keeps the previous artifact's cached file. That's the intended cache behavior, but nothing is logged, so a feed that has been failing for weeks is indistinguishable from a healthy one without manually diffing the deployed XML. There is no feed.bozo / status check anywhere.

2. Empty-feed overwrite: if the fetch succeeds but every entry fails per-entry parsing (e.g. an upstream date-format change — both strptime formats at src/rss.py:45-47 raise, each exception is caught and only logged), the loop appends zero items and new_feed.write still overwrites the previously good cached XML with an empty feed (src/rss.py:60-61). The failure mode thus destroys the cached data instead of preserving it.

Related: the workflow's verification step (.github/workflows/build.yml:72-77) checks only the SQLite snapshots and schemas; RSS outputs are never verified, so case 2 deploys unnoticed.

Suggested fix

• Log a warning when feed.bozo is set or feed.entries is empty, e.g. ::warning:: so it surfaces in the Actions summary.
• Skip writing (keep the cached file) when entries were fetched but zero items survived parsing.
• Optionally extend the verify step to check that build/rss/*.xml exists and is non-empty.