After a VAPID keypair rotation, subscribers minted under the old key
silently stopped receiving notifications while the UI still reported
"on", and the dead rows were never cleaned up.
Frontend (enable): a browser holding an existing PushSubscription never
re-subscribed, so it kept using the old applicationServerKey. Now the
current server VAPID key is always fetched and compared against the
subscription's baked-in key; on a mismatch the stale subscription is
unsubscribed and re-created with the new key. The matching-key path is
unchanged.
Backend (send): a rotated key makes the push service reject delivery
with 401/403, which returned "error" and left the row in place forever.
Treat 401/403 as permanently dead alongside 404/410 so the caller prunes
the row. Genuinely transient failures (rate limits, 5xx) still return
"error" and keep the row.
Also correct the default VAPID contact to mailto:admin@thermograph.org
(the .app domain was a typo); the env override is unchanged.
Extends tests/notifications/test_push.py with the send() status mapping
and the contact default.
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