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>

backend/app/routers/library.py

@@ -18,6 +18,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.database import get_db
 from app.models import Photo
+from app.models.folders import SourceRoot
 from app.models.user import User
 from app.dependencies import get_current_user
 
@@ -105,6 +106,13 @@ async def get_library_stats(
 
     size = (await db.execute(select(func.sum(Photo.file_size)).where(owner))).scalar() or 0
 
+    # Source root directories (active ones only).
+    roots = (
+        await db.execute(
+            select(SourceRoot.path).where(SourceRoot.is_active.is_(True)).order_by(SourceRoot.path)
+        )
+    ).scalars().all()
+
     return {
         "all_photos": all_photos_count,
         "rated": rated_count,
@@ -116,6 +124,7 @@ async def get_library_stats(
         "total_videos": video_count,
         "total_size": size,
         "total_size_gb": round(size / (1024**3), 2) if size else 0,
+        "source_dirs": roots,
     }
 
 @router.post("/scan")