feat(web): a start that waits for the display answers 202 and is delivered in the background

The start route held a request open for up to 45 s while a cold-started
display loaded its plugins; the MQTT bridge (15 s timeout) and browsers
reported a failure for a request that was then delivered.

Now, when no display is listening, the route starts the service if asked
and answers 202 with status "starting" at once. A single worker in the web
process (web_interface/on_demand_dispatch.py) sends the request until the
display acknowledges it or the wait runs out (45 s cold start, 10 s for a
running service without a socket yet). A newer start supersedes the
pending one; a stop cancels it (and succeeds, with cancelled_request_id,
even with no display listening). The outcome is reported by
/display/on-demand/status (source "web": starting, or error with
start-timeout or the socket's reason, until the display publishes
something newer) and by /display/current-status as on_demand_pending.

Callers: the web UI's on-demand modal and "Preview on display" treat
"starting" as taken (an info toast); the MQTT bridge already treats any
non-error 2xx as success (now pinned by a test).

Tests: the dispatcher (ack, retry then ack, start-timeout, other failures,
superseded, an in-flight ack for a superseded start, stop while pending,
a per-start wait, outcome lifetime); the routes (202, status routes while
pending and after a timeout, a later display state replacing the failure,
stop while pending, a new start superseding); a JS suite for app.js.
Mutation check: 20 mutants on the worker, the routes and app.js, 20 killed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/test_api_v3_on_demand_restart.py

@@ -16,7 +16,8 @@ This file previously pinned that restart path (it guarded a broken
 ``import _pkg.time`` inside it). The path is gone; these tests pin its
 replacement: a running service is left alone, a stopped one is started (only
 when start_service is set), and the request goes over the control socket
-either way -- to a stopped display once it has started and its socket is up.
+either way -- to a stopped display once it has started and its socket is up,
+sent by the web process's dispatcher after the route has answered 202.
 Nothing is ever written to the cache: the file mailbox is gone (stage 5).
 
 The service helpers are patched where they run. display.py binds
@@ -26,6 +27,7 @@ _run_systemctl_command is the one place a systemctl command is issued.
 """
 
 import sys
+import time
 from pathlib import Path
 from unittest.mock import patch
 
@@ -157,10 +159,18 @@ class TestStartWhileTheServiceIsStopped:
     def test_start_service_starts_it_once_and_never_stops_it(self, api_v3_client, service):
         service["state"]["active"] = False
         response = api_v3_client.post(START_URL, json={"plugin_id": "weather"})
-        assert response.status_code == 200, response.get_json()
+        # Answered at once: the web process sends the request in the
+        # background once the started display listens.
+        assert response.status_code == 202, response.get_json()
+        assert response.get_json()["status"] == "starting"
         assert _systemctl_verbs(service["systemctl"]) == ["start"]
         service["stop_service"].assert_not_called()
-        # Sent once the started display's socket answered.
+        from web_interface import on_demand_dispatch
+        dispatcher = on_demand_dispatch.current()
+        deadline = time.monotonic() + 5
+        while dispatcher.pending() and time.monotonic() < deadline:
+            time.sleep(0.01)
+        assert dispatcher.status()["status"] == "delivered"
         assert [s[0] for s in service["sent"]] == ["start"]
         assert _mailbox_writes(service["cache"]) == []