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/actions/nearBottom.ts

@@ -0,0 +1,70 @@
+/**
+ * Fires `onHit` whenever the attached element scrolls near the bottom of
+ * its scroll container. Mirrors PhotoPrism's infinite-scroll trigger from
+ * `frontend/src/page/photos.vue`: an IntersectionObserver on a sentinel
+ * div with a rootMargin equal to ~4 viewport heights, so the next page is
+ * fetched well before the user actually reaches the end.
+ *
+ * Usage: attach to a sentinel <div /> placed at the bottom of the scroll
+ * area. The host gates calls via `enabled` (= `hasNextPage && !isFetching`).
+ *
+ *   <div use:nearBottom={{ onHit: fetchNextPage, enabled: canFetch }} />
+ */
+export interface NearBottomParams {
+	onHit: () => void;
+	/** When false the observer ignores intersections (use for the
+	 *  hasNextPage + !isFetchingNextPage gate). */
+	enabled?: boolean;
+	/** Pre-load distance in pixels. PhotoPrism uses `innerHeight * 4`;
+	 *  we default to the same. Caller can pass a number for tests. */
+	preloadPx?: number;
+	/** Optional scroll root (defaults to the viewport). Pass the
+	 *  scrolling ancestor when the page itself doesn't scroll, which is
+	 *  our case — the timeline scrolls inside `<main>`. */
+	root?: Element | null;
+}
+
+export function nearBottom(node: HTMLElement, params: NearBottomParams) {
+	let current: NearBottomParams = params;
+	let io: IntersectionObserver | null = null;
+
+	function buildObserver(p: NearBottomParams) {
+		io?.disconnect();
+		const preload = p.preloadPx ?? Math.max(800, window.innerHeight * 4);
+		io = new IntersectionObserver(
+			(entries) => {
+				if (!current.enabled) return;
+				for (const e of entries) {
+					if (e.isIntersecting) {
+						current.onHit();
+						return;
+					}
+				}
+			},
+			{
+				root: p.root ?? null,
+				// Inflate the root's bottom edge so we trip well before
+				// the sentinel actually enters the viewport.
+				rootMargin: `0px 0px ${preload}px 0px`
+			}
+		);
+		io.observe(node);
+	}
+
+	buildObserver(current);
+
+	return {
+		update(next: NearBottomParams) {
+			const rootChanged = next.root !== current.root;
+			const preloadChanged = next.preloadPx !== current.preloadPx;
+			current = next;
+			// `enabled` and `onHit` are read live inside the callback,
+			// so they don't require rebuilding the observer. Root and
+			// preloadPx are baked in at construction.
+			if (rootChanged || preloadChanged) buildObserver(current);
+		},
+		destroy() {
+			io?.disconnect();
+		}
+	};
+}