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

CLAUDE.md

@@ -82,6 +82,8 @@ sources (Tasmota/HA/MQTT/manual/CSV)
 
 **Time & DST (SDD §10):** store UTC everywhere; bucket and display in the instance timezone (default `Europe/Berlin`). "Daily cost" boundaries are local-midnight — use `time_bucket(..., 'Europe/Berlin')`.
 
+**In-app update (`UpdateRunner`):** the dashboard shows a banner when a newer tag exists (`UpdateCheckService`, cached, never blocks a render). Triggering an update is **off by default** and needs three independent conditions: `MeterVault__AllowInAppUpdate`, at least one configured API key, and a caller presenting one (constant-time compare). `AllowAnonymousApi` deliberately cannot reach it — the updater builds whatever is on the branch and the LXC runs the app as root, so this is root-equivalent. Launches detached via `systemd-run` because the update restarts the service. Treat any change here as security-critical; `UpdateRunnerTests` pins the gate.
+
 **Secrets (SDD §6.4):** broker/HA tokens are **never** stored in DB plaintext. Two forms, chosen per connector in the admin UI: a *reference* (`token_env`/`password_env` naming an env var or Docker secret path) resolved at runtime, or *encrypted at rest* (`token_enc`/`password_enc`) via `SecretProtector` over the ASP.NET Core data-protection key ring. Exactly one survives a save; `EndpointSecret.Resolve` is the single resolution path (encrypted wins). The key ring lives outside the app directory (`MeterVault__DataProtectionKeyPath`, default `/var/lib/metervault/keys`) because the LXC updater republishes `/opt/metervault`. `ExportService` drops `*_enc` values — they are bound to the originating key ring.
 
 ## Reference-data behaviours the code must reproduce (from `sampledata/`)