fix(gnss): date replayed snapshots by the proxy-reported age, not cross-clock fetchedAt

The layer compared the server's fetchedAt with the browser's clock, so a
skewed client clock could shift or void the 30-minute window. The proxy now
returns ageMs (its own now minus fetchedAt, at serve time, on fresh, cached
and stale answers); the source refuses a snapshot without a finite,
non-negative ageMs, and the layer dates rows as its now() - ageMs. fetchedAt
stays in the payload for display.

Tests: proxy cached/stale answers report ageMs on the server clock; the
source rejects missing/invalid/negative ageMs; the layer replay test drives
ageMs. qa-gnss stubs send ageMs.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

CHANGELOG.md

@@ -169,7 +169,8 @@
   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 (with its
-  original observation time, for at most 30 minutes) when the upstream fails.
+  original observation time and `ageMs`, 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