Add maintenance mode to silence alerts while working on a server or integration

Rebooting Proxmox or patching a server triggered failure/offline alerts
you then had to dismiss. A maintenance window silences alerts about one
server, integration, or DNS provider for a chosen time. New Maintenance
page (start with a duration and optional reason, end early, see what's
silenced and what isn't) and a banner in the app shell so every signed-in
user can see what is currently silenced. Starting/ending is operator-only
and audit-logged; starting one on a target that already has a window
restarts its clock instead of stacking.

Silenced for the target: server offline/disk alerts, Proxmox/Synology
storage and health alerts, Proxmox backup alerts, and "integration down"
alerts. Not silenced: expiry and update reminders, DNS change notices.

The design goal is that this cannot hide a real outage:
- Every window has a required end (5 min to 7 days); there is no
  open-ended option, so a forgotten window expires by itself.
- A silenced problem is deliberately NOT recorded as "known". If it is
  still present when the window ends it alerts then, as new. A problem
  that was already alerted before the window stays known, so it isn't
  repeated, and is reported cleared only after the window ends.
- Failure alerts keep counting failures during a window without marking
  themselves alerted, so an outage that outlasts the window alerts on the
  very next failed call.

Known limitation, stated on the page: integration-failure alerts are
tracked per service TYPE (all "proxmox"), not per configured instance, so
a window on one Proxmox integration also silences a failure on a second
Proxmox integration while it's open. Fixing that means threading the
integration id through every adapter and the diagnostic log, which is a
much larger change than this feature.

Also moved the API-error-message helper out of Secrets.tsx into a shared
util now that two pages use it. New table maintenance_windows (migration
0008).

Verified with 44 checks: the condition-key-to-subject mapping (including
server:3 vs server:33), the diff rules with silenced subjects (new problem
not recorded, alerts when the window ends; already-known one carried and
not repeated; clears only after the window), window expiry and
integration/DNS-provider source matching, the failure tracker end to end
against a webhook (silent during a window while an unrelated service still
alerts; outage that outlasts the window alerts on the next failure and
only once; fail-and-recover fully inside a window sends nothing), a full
health pass against a real window, and the real router with a stubbed
session (role rules, duration bounds including the missing-duration case,
extend-not-stack, 404s, deleted targets hidden, audit entries). Real dev
database mtime untouched.

Not done: I haven't clicked through the new page or banner in a browser
(they sit behind the Authentik login); it builds and the API behind it is
tested.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -142,6 +142,14 @@ raise one notification when the problem starts and one when it clears (both
 thresholds are set under Settings → Notifications). Active problems are
 remembered across restarts, so a rebuild doesn't re-alert them.
 
+**Maintenance mode** silences alerts about one server, integration, or DNS
+provider while you work on it (server offline / disk, storage and Synology
+health, Proxmox backup alerts, and "integration down" for that service type).
+Every window has a fixed end (5 minutes to 7 days) and expires on its own, and
+a problem that began during a window and is still present when it ends alerts
+then — a forgotten window can't hide an outage. A banner shows what's currently
+silenced to every signed-in user.
+
 The app is installable as a PWA — "Install app" / "Add to Home Screen" from the
 browser gives it its own icon and a standalone window on phone or desktop. This
 needs the site to be served over HTTPS (browsers only offer install on secure