fix(gnss): age degraded evidence and replayed snapshots on their own clocks

Maintainer review on #855 (mazy1998, 2026-10-06):

- records: presence (seenAt) and degraded evidence (degradedAt) age
  separately, so continuing healthy reports no longer keep an older
  degraded flag alive past 30 minutes; an aircraft returning after the
  window starts fresh. pruneGnssObservations ages the store with no new
  rows.
- Rows are dated by the snapshot's fetchedAt, which the proxy keeps on
  cached and stale answers: a replay never restarts the window, the
  proxy stops serving a stale snapshot older than the window, the source
  refuses an undated one, and cells expire during fetch failures and
  while there is no view anchor.
- Normalization accepts ADS-B versions 1 and 2 only; a missing, invalid
  or negative seen_pos is not a confirmed position from the last minute.
- qa-gnss: keyboard, 390x844 viewport (fresh page: isMobile reloads) and
  replay-age sections; the live cell assertion bins the returned rows
  with the layer's own rules instead of a raw row count.

CHANGELOG.md

@@ -168,8 +168,12 @@
   machine-readable provenance (`provenance`, `classifier: gev-nic-nacp-v1`).
   Keyless, worldwide wherever adsb.lol has receivers; reads
   a capped 250 nm snapshot around the camera through `/api/gnss-integrity`,
-  which returns integrity fields only and serves the last snapshot when the
-  upstream fails.
+  which returns integrity fields only and serves the last snapshot (with its
+  original observation time, for at most 30 minutes) when the upstream fails.
+  Only ADS-B versions 1 and 2 with a known position age of 0-60 s count. A
+  degraded report keeps an aircraft degraded for 30 minutes after that report,
+  not after its latest healthy one; replayed snapshots keep their age, and
+  cells expire on schedule during fetch failures or without a view anchor.
 
 ## [0.2.1] — 2026-10-02 — God's Eye View through Your Agent (with security fixes and other features)