Notify a phone that has this installation installed

A `webpush` alert channel, and the PWA it needs to arrive. The payload is
encrypted to the subscription (RFC 8291) and the request signed with this
installation's own keypair (RFC 8292), both over `http-ece` — `pywebpush`
does the same in one call but brings `requests` and `aiohttp` with it, two
HTTP stacks beside httpx on a machine that may be a Raspberry Pi.

The manifest and the worker are hand-written rather than `vite-plugin-pwa`:
there is nothing worth precaching when the page carrying the credential is
`no-store`, so the worker handles `push` and `notificationclick` and nothing
else. `registration.scope` is the app's root in both places it runs, which is
why the payload carries no URL.

A run finishing in error is the first event worth waking someone for; `ok`
and `cancelled` describe to nothing, so a nightly batch that works stays
quiet. The events were already on the bus — only the filter changed.

`WEBPUSH_FILE` is a derived path, so the keypair lands on the data volume
with the alerts beside it. Off it, a rebuild would silently stop every phone
being notified: the key they subscribed against would be gone.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014EbeFPm6WNC3YD9vrqqT3a

backend/tests/flow/test_alerts.py

@@ -188,6 +188,13 @@ def test_alerting_can_be_switched_off():
             "The work queue is unreachable",
         ),
         ({"type": "engine_degraded", "reason": "lag"}, "The engine is struggling"),
+        # Every run ends. Only a failed one is worth a phone buzzing.
+        (
+            {"type": "run_finished", "flow": "nightly", "run": 7, "status": "error"},
+            "A run of 'nightly' failed",
+        ),
+        ({"type": "run_finished", "flow": "nightly", "status": "ok"}, None),
+        ({"type": "run_finished", "flow": "nightly", "status": "cancelled"}, None),
         ({"type": "node_health", "health": "ok"}, None),
         # The engine publishes `health`, not `status`: reading the wrong key
         # meant a device dropping never alerted anyone.