feat: PhotoPrism M0 bring-up — compose stack, web client, sidecar, migrate

Replace the legacy mule-image backend with PhotoPrism plus a thin
SvelteKit client and a Node sidecar for endpoints PhotoPrism doesn't
expose (file rename), and add a two-phase migrator (metadata via PUT,
heaps → albums) for the existing Postgres library.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/stores/preview.svelte.ts

@@ -0,0 +1,36 @@
+/**
+ * Single-photo preview overlay state. The lightbox sits in +layout.svelte
+ * and listens to this store; any view (timeline, duplicates view, heaps)
+ * can `open(uid)` to pop it. The order array is mirrored from whatever
+ * list the user is currently looking at so prev/next stay in context.
+ */
+export const preview = $state<{
+	uid: string | null;
+	order: string[];
+}>({
+	uid: null,
+	order: []
+});
+
+export function openPreview(uid: string, order?: string[]): void {
+	if (order) preview.order = order;
+	preview.uid = uid;
+}
+
+export function closePreview(): void {
+	preview.uid = null;
+}
+
+export function previewNext(): void {
+	if (!preview.uid) return;
+	const i = preview.order.indexOf(preview.uid);
+	if (i < 0 || i >= preview.order.length - 1) return;
+	preview.uid = preview.order[i + 1];
+}
+
+export function previewPrev(): void {
+	if (!preview.uid) return;
+	const i = preview.order.indexOf(preview.uid);
+	if (i <= 0) return;
+	preview.uid = preview.order[i - 1];
+}