feat(host): warn about low disk before it becomes unrecoverable

Storage visibility existed in exactly one place: a passive Speicherauslastung widget
on the ADMIN dashboard. A host who isn't the admin had no view of it, and nothing
warned anyone. README carried "Low-disk alert (< 10 GB free)" under Planned since v1.

Two things make this a safety net rather than a nice-to-have. postgres_data,
media_data and exports_data are all Docker named volumes on ONE filesystem, so
running out doesn't degrade a subsystem -- Postgres stops being able to write and the
whole event goes down. And the keepsake needs room for two gallery-sized archives,
which the export preflight can only ever refuse AFTER the release, when the event is
over and every remedy is harder.

So the threshold is not a fixed number alone. It fires on the 10 GB floor the README
always named, OR on "you could not build the keepsake right now" -- the trigger a
host can still act on, computed with the same arithmetic the preflight uses. Unknown
free space is NOT low: it fails open like the upload quota and the preflight do,
because a banner that cries wolf on an unreadable mount is a banner nobody reads.

Carried on GET /host/event, which the dashboard already fetches on load and on every
reload -- no new endpoint, no new poll. Rendered above everything else including the
PIN-reset queue, and it names the consequence (the event, not just the download)
rather than only the number.

Also fixes the host page's formatBytes, which topped out at MB: 30 GB free would have
rendered as "30720.0 MB", and a guest with 2 GB of uploads was already being shown
that way in the user list.

Tests: 5 unit on the threshold (including that plenty of free space is still low when
the keepsake wouldn't fit -- the case a fixed threshold misses entirely), 3 e2e.
The e2e drives it through `original_size_bytes` rather than a genuinely full disk:
the estimate is pure SQL over that column, so overstating one row moves the
accounting without touching a byte on disk.

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

frontend/src/routes/host/+page.svelte

@@ -28,6 +28,9 @@
 		is_active: boolean;
 		uploads_locked: boolean;
 		export_released: boolean;
+		disk_free_bytes: number | null;
+		keepsake_required_bytes: number;
+		disk_low: boolean;
 	}
 
 	interface PinResetRequest {
@@ -395,7 +398,11 @@
 	function formatBytes(bytes: number): string {
 		if (bytes < 1024) return `${bytes} B`;
 		if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`;
-		return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
+		// GB matters here now that this also renders free disk and keepsake size — the previous
+		// version topped out at MB, so 30 GB free read as "30720.0 MB" (and a guest with 2 GB of
+		// uploads was already being rendered the same way in the user list).
+		if (bytes < 1024 * 1024 * 1024) return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
+		return `${(bytes / (1024 * 1024 * 1024)).toFixed(1)} GB`;
 	}
 </script>
 
@@ -507,6 +514,38 @@
 				{error}
 			</div>
 		{:else if event}
+			<!-- ── Speicherwarnung ─────────────────────────────────────────────
+			     Above everything else on purpose. All three volumes (postgres_data, media_data,
+			     exports_data) sit on one filesystem, so running out doesn't degrade a subsystem —
+			     it stops Postgres writing and takes the event down. And the keepsake needs room
+			     for TWO gallery-sized archives, which is only actionable BEFORE the release: the
+			     export preflight can say "this didn't fit", but by then the event is over and the
+			     remedies are all much harder.
+
+			     Only the admin dashboard had any storage visibility at all, and a host is often
+			     not the admin. `disk_low` fails closed to "not low" on an unreadable mount, so
+			     this cannot cry wolf. -->
+			{#if event.disk_low && event.disk_free_bytes !== null}
+				<div
+					class="rounded-xl border border-red-300 bg-red-50 p-4 dark:border-red-800 dark:bg-red-950/30"
+					data-testid="low-disk-warning"
+				>
+					<h2 class="font-semibold text-red-900 dark:text-red-200">Speicherplatz wird knapp</h2>
+					<p class="mt-1 text-sm text-red-800 dark:text-red-300">
+						Noch <strong>{formatBytes(event.disk_free_bytes)}</strong> frei.
+						{#if event.keepsake_required_bytes > event.disk_free_bytes}
+							Für das Keepsake werden derzeit ca.
+							<strong>{formatBytes(event.keepsake_required_bytes)}</strong> benötigt — es kann
+							momentan <strong>nicht</strong> erstellt werden.
+						{/if}
+					</p>
+					<p class="mt-1.5 text-xs text-red-700 dark:text-red-400">
+						Schaffe Speicher frei oder vergrößere den Datenträger. Wenn der Datenträger vollläuft,
+						fällt das gesamte Event aus — nicht nur der Download.
+					</p>
+				</div>
+			{/if}
+
 			<!-- ── PIN-Reset-Anfragen ──────────────────────────────────────── -->
 			{#if pinResetRequests.length > 0}
 				<div