feat: show source directories in library stats and settings UI

Add active source root directories to the library stats endpoint and
display them in the settings page. Hardcode container PHOTO_DIRS to
/photos since the volume mount handles host path mapping. Add .env to
.gitignore to prevent committing secrets.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -34,6 +34,7 @@ dist-ssr/
 .DS_Store
 
 # Environment
+.env
 .env.local
 .env.*.local