feat(video): add video upload with remux/transcode playback

Add video as a media kind in photos: streaming upload with per-kind
limits, ffprobe metadata, poster frame reusing the image variant
pipeline, and a playback object (remux when web-compatible, else
h264/yuv420p/aac transcode). Video jobs run on a separate pool with a
long lease. Frontend gains a video viewer, grid badge/duration and
video upload.

frontend/src/lib/components/Viewer.svelte

@@ -60,6 +60,11 @@
 	const panoTag = $derived(isPanoTag(current) || isPanoTag(fresh));
 	const pano = $derived(panoTag && !panoFailed);
 	const panoSrc = $derived(fullSrc || previewSrc);
+	const isVideo = $derived((current?.kind ?? fresh?.kind) === 'video');
+	const playbackSrc = $derived(fresh?.playback_url ?? current?.playback_url ?? '');
+	const videoPoster = $derived(
+		fresh?.preview_url ?? fresh?.thumb_url ?? current?.preview_url ?? current?.thumb_url ?? ''
+	);
 
 	$effect(() => {
 		const id = current?.id;
@@ -268,6 +273,10 @@
 
 	function onKey(ev: KeyboardEvent) {
 		if (isTyping()) return;
+		// Let the native video player own its playback keys.
+		if (isVideo && (ev.code === 'Space' || ev.key === 'ArrowLeft' || ev.key === 'ArrowRight')) {
+			return;
+		}
 		if (ev.key === 'Escape') {
 			if (!document.fullscreenElement) onClose();
 			return;
@@ -396,6 +405,41 @@
 					onFallback={() => (panoFailed = true)}
 				/>
 			</div>
+		{:else if isVideo}
+			<div
+				class="bg-canvas flex min-h-0 flex-1 items-center justify-center overflow-hidden"
+				role="group"
+				aria-label="Просмотр видео"
+				oncontextmenu={onContextMenu}
+			>
+				{#if playbackSrc}
+					<!-- svelte-ignore a11y_media_has_caption -->
+					<video
+						class="m-auto block max-h-full max-w-full"
+						src={playbackSrc}
+						poster={videoPoster}
+						controls
+						playsinline
+						preload="metadata"
+					></video>
+				{:else}
+					<div class="relative flex items-center justify-center">
+						{#if videoPoster}
+							<img
+								src={videoPoster}
+								alt={current?.original_filename ?? ''}
+								class="max-h-full max-w-full object-contain"
+							/>
+						{/if}
+						<span
+							class="absolute rounded bg-black/60 px-2 py-1 text-xs text-white"
+							role="status"
+						>
+							Обработка видео…
+						</span>
+					</div>
+				{/if}
+			</div>
 		{:else}
 			<div
 				bind:this={scroller}