Add notification quiet hours with digest delivery

Instant notifications (DNS changes, integration failure/recovery
alerts) had no way to avoid pinging overnight. Adds a "Quiet hours"
window under Settings -> Notifications: notifications that would fire
during the window are held in a new notification_queue table instead
of sent immediately, then delivered as one combined digest at the end
time (in the same timezone already used for the daily checks) via a
new scheduled flush job. The scheduled daily checks (secret expiry,
Tailscale key, Docker updates, Proxmox backups) already only fire
once at a chosen time, so this mainly matters for the instant ones.
Includes a live "N queued" indicator with a manual "Flush now" button
for visibility, and correctly falls back to sending immediately
whenever the feature is disabled (the default).

Gating lives at the single choke point every notification already
flows through (notify()), so no per-event-type wiring was needed.

Verified against a fake webhook receiver on an isolated scratch
database: exhaustively checked the midnight-wraparound window math
(9 cases including exact-boundary inclusive/exclusive edges) against
synthetic "now" values rather than depending on when the test
happens to run, then end-to-end through the real notify()/
flushQuietHoursQueue() functions with a window constructed around the
actual current time — confirmed a notification during the window
queues instead of sending, the flush produces one digest with the
original title/message intact and clears the queue, a notification
outside the window sends immediately, and disabling the feature
entirely sends immediately regardless of the window. Confirmed the
real dev database's mtime was untouched throughout.

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

server/drizzle/0006_aberrant_darkhawk.sql

@@ -0,0 +1,6 @@
+CREATE TABLE `notification_queue` (
+	`id` integer PRIMARY KEY AUTOINCREMENT NOT NULL,
+	`title` text NOT NULL,
+	`message` text NOT NULL,
+	`created_at` text DEFAULT (current_timestamp) NOT NULL
+);