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>

frontend/src/services/api.ts

@@ -625,6 +625,7 @@ export interface LibraryStats {
   total_videos: number
   total_size: number
   total_size_gb: number
+  source_dirs: string[]
 }
 
 // Heaps API