feat(notifications): Web Push delivers notifications to every device, even with TREK closed

A fourth built-in channel next to email, webhook and ntfy. Browsers
register per device, the server encrypts per RFC 8291 and signs per RFC
8292 with node:crypto and sends only to known push services through the
SSRF guard. The event matrix decides per category what arrives by push;
admins switch the channel on under Notifications.

VAPID keys come from the environment or are generated once, only while
neither stored row exists, and kept encrypted. A pair that does not
decrypt or match is never overwritten and never swapped for another one:
push stays off with an error in the log and every device stays
registered until the key is fixed. The subject is VAPID_SUBJECT, then an
https APP_URL, then the project URL.

Only chat messages collapse into one notification per trip, and a
replacement still alerts. A 403 removes a device only after three
failures in a row; 404 and 410 remove it at once. Password change, reset
and an admin setting a password drop the user's devices with the other
credentials. The request log redacts subscriptions and endpoints, boot
errors print *** for every secret variable, and logout forgets the
device with a plain fetch so an expired session cannot redirect midway.

Addresses discussion #894.

server/.env.example

@@ -68,6 +68,10 @@ DEMO_MODE=false # Demo mode - resets data hourly
 
 # LLM_TIMEOUT_MS=900000 # Ceiling (ms) for one AI-parsing call before it is abandoned. One value for every provider, and the underlying HTTP client is held to it too. Generous by default so heavier parsing work fits without a code change; lower it on a cloud provider to fail fast. (default: 900000)
 
+# VAPID_PUBLIC_KEY= # Web Push: the public half of the key pair browsers subscribe with (base64url, 65 bytes). Leave both halves unset and TREK generates a pair on first start and keeps it in the database, so it travels with every backup. Set both to use a pair of your own, e.g. printed by `npx web-push generate-vapid-keys`. Changing the pair later means every device has to turn push on again.
+# VAPID_PRIVATE_KEY= # Web Push: the private half (base64url, 32 bytes). Only used together with VAPID_PUBLIC_KEY; a pair whose halves do not belong together, or only one half, turns push off with an error in the log until the pair is fixed. A pair stored in the database is not used in its place, and every device stays registered.
+# VAPID_SUBJECT=mailto:admin@example.com # Web Push: the contact push services see with every message, a mailto: address or an https:// URL. Unset, TREK uses APP_URL when it is https, else the TREK project page.
+
 # UNSPLASH_ACCESS_KEY= # Optional Unsplash Access Key for trip-cover and place-image search. Without one, TREK uses Unsplash's unauthenticated endpoint, which some datacenter/VPS IPs get blocked from (#1449). Get a free key at unsplash.com/developers. Overrides any key set per-admin in Admin > Settings. Can also be configured there instead of here.
 
 # Initial admin account — ONLY applied on the first boot, when the database has no