repo cleanup: retire legacy mule-image stack, lock PhotoPrism UI to loopback

Delete the old Python+React mule-image stack (backend/, frontend/,
docker-compose.yml, mulita.yml, .env*) plus the one-shot migration and
sample dirs (migrate/, photos-sample/, photovault-app-prompt.md). Only
the PhotoPrism + Go sidecar + SvelteKit web stack remains, so drop the
".photoprism." qualifier from the compose+env filenames.

Bind PhotoPrism's port to 127.0.0.1 so the user-facing surface is just
the SvelteKit web/ app; admin reaches PP's UI via SSH tunnel. Flatten
PHOTOPRISM_INDEX_WORKERS' nested default (podman-compose's interpolator
doesn't expand ${A:-${B:-…}}). Rewrite README for the current stack.

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

.gitignore

@@ -37,7 +37,6 @@ dist-ssr/
 .env
 .env.local
 .env.*.local
-.env.photoprism
 
 # Database
 *.db
@@ -66,9 +65,6 @@ docker-compose.override.yml
 /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/
 
@@ -80,4 +76,3 @@ docker-compose.override.yml
 
 # Thumbnails
 /thumbs/
-/trash/backend/yolov8n.pt