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/services/batch.ts

@@ -0,0 +1,50 @@
+/**
+ * Fan-out helper used wherever PhotoPrism lacks a true batch endpoint.
+ * Bounded concurrency keeps the indexer happy on slow hosts; each item's
+ * result is collected and the aggregate `{updated, errors[]}` mirrors the
+ * shape the legacy mule-image bulk endpoint returned, so existing toast
+ * + undo plumbing slots in without changes.
+ */
+
+export interface BatchResult<T> {
+	updated: T[];
+	errors: { id: string; message: string }[];
+}
+
+export interface BatchOptions {
+	concurrency?: number;
+	onProgress?: (done: number, total: number) => void;
+}
+
+export async function batchEdit<T>(
+	ids: string[],
+	fn: (id: string) => Promise<T>,
+	opts: BatchOptions = {}
+): Promise<BatchResult<T>> {
+	const concurrency = Math.max(1, opts.concurrency ?? 8);
+	const updated: T[] = [];
+	const errors: { id: string; message: string }[] = [];
+	let i = 0;
+	let done = 0;
+
+	async function worker() {
+		while (true) {
+			const idx = i++;
+			if (idx >= ids.length) return;
+			const id = ids[idx];
+			try {
+				updated.push(await fn(id));
+			} catch (err) {
+				errors.push({ id, message: err instanceof Error ? err.message : String(err) });
+			} finally {
+				done++;
+				opts.onProgress?.(done, ids.length);
+			}
+		}
+	}
+
+	await Promise.all(
+		Array.from({ length: Math.min(concurrency, ids.length) }, () => worker())
+	);
+	return { updated, errors };
+}