Update: add an opt-in "Update now" button, gated on an API key

Adds the button, plus POST /api/v1/system/update for driving it from Home
Assistant or curl. Both pull the latest source, rebuild and restart the service.

The gating is the substance of this change. The updater builds whatever is on
the branch and the LXC runs MeterVault as root, so triggering it is
root-equivalent on that host, and the web UI has no login — "reachable from the
dashboard" alone would mean any device on the network could take the machine.
Three independent conditions must hold before anything runs: the operator set
MeterVault__AllowInAppUpdate, at least one API key is configured, and the caller
presented one, compared in constant time so retries cannot time out the key.

AllowAnonymousApi cannot reach it. That flag opens reads, and opening reads must
not open root, so the endpoint checks the presented key itself rather than
relying on the shared group filter that honours it. Availability is re-checked
inside LaunchAsync rather than trusting the caller to have done so.

The UI button asks for the key every time instead of remembering it: with no
login, a browser left open on the dashboard would otherwise be a standing
permission to execute code on the host. The key is cleared from component state
immediately, and a wrong key and a keyless deployment give the same message so
an unauthenticated caller cannot tell them apart.

Launched detached through systemd-run: the updater restarts the service, so a
child process would be killed part-way through, leaving the app down with a
half-published build. --collect reaps the transient unit so a later update is
not blocked by the remains of the previous one.

Off by default, and where there is no /usr/bin/update — a container, a dev box —
it reports that rather than half-running something. Tests pin every refusal,
including through the real HTTP pipeline; none of them launch anything.

Claude-Session: https://claude.ai/code/session_01V6joyergfvVLFEizH1hJLd

tests/Integration.Tests/ApiTests.cs

@@ -11,6 +11,28 @@ public sealed class ApiTests(TimescaleFixture fx)
 {
     private sealed record ReadingPush(int MeterId, DateTimeOffset Time, double Value);
 
+    [Fact]
+    public async Task Update_endpoint_is_shut_by_default_and_not_openable_anonymously()
+    {
+        // Through the real pipeline, not just the runner: an install that never opted in must return
+        // a refusal for this endpoint even when the caller presents a valid key, and even when the
+        // API itself has been opened anonymously. 409 rather than 401 — the endpoint is disabled,
+        // which is a different fact from the caller being unauthenticated.
+        using var factory = new MeterVaultAppFactory(fx.ConnectionString, configureApiKey: true);
+        using var client = factory.CreateClient();
+
+        using var withKey = new HttpRequestMessage(HttpMethod.Post, "/api/v1/system/update");
+        withKey.Headers.Add("X-Api-Key", MeterVaultAppFactory.ApiKey);
+        var authorised = await client.SendAsync(withKey);
+        Assert.Equal(HttpStatusCode.Conflict, authorised.StatusCode);
+
+        // And without a key it is certainly not reachable.
+        var anonymous = await client.PostAsync(new Uri("/api/v1/system/update", UriKind.Relative), content: null);
+        Assert.True(
+            anonymous.StatusCode is HttpStatusCode.Unauthorized or HttpStatusCode.Conflict,
+            $"expected the update endpoint to refuse an unauthenticated caller, got {anonymous.StatusCode}.");
+    }
+
     [Fact]
     public async Task Readings_push_requires_key_and_writes_when_authorized()
     {