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>

sidecar/README.md

@@ -37,9 +37,9 @@ The sidecar is a `sidecar` service in the PhotoPrism compose stack.
 Bringing the whole stack up brings it up too:
 
 ```sh
-podman-compose --env-file .env.photoprism \
-  -f docker-compose.photoprism.yml \
-  -f docker-compose.photoprism.podman.yml \
+podman-compose --env-file .env \
+  -f docker-compose.yml \
+  -f docker-compose.podman.yml \
   up -d
 ```