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>

.gitignore

@@ -37,6 +37,7 @@ dist-ssr/
 .env
 .env.local
 .env.*.local
+.env.photoprism
 
 # Database
 *.db
@@ -61,6 +62,16 @@ build/
 # Docker
 docker-compose.override.yml
 
+# PhotoPrism state (sidecars, cache, thumbs, db backups) — regenerable.
+/pp/storage/
+/pp/import/
+
+# Local-dev sample photo library used for M0 PhotoPrism validation.
+/photos-sample/
+
+# Sidecar runtime state (per-user marks etc.) — generated, not seed data.
+/sidecar/data/
+
 # Photos (for development)
 /photos/