fix: vision tasks inherit user_id, admin owns mount root

- detect_objects, classify_content, recluster_faces now look up the
  photo's user_id and set it on created Tag rows — fixes tags being
  invisible to the owning user due to NULL user_id
- Initial admin setup creates source root at the mount root (/photos)
  instead of a subdirectory, since the admin owns the entire library
- Revert to OpenCLIP ViT-B/32 (512-d) as default embedder — SigLIP
  requires transformers version alignment not yet available in the
  Docker image. SigLIP2 code remains for future enablement.
- Add transformers to requirements for future SigLIP support

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

backend/app/tasks/vision.py

@@ -163,6 +163,12 @@ def detect_objects(photo_id: str):
 
     session = _get_sync_session()
     try:
+        # Get the photo's user_id so tags inherit ownership.
+        photo = session.execute(
+            select(Photo).where(Photo.id == photo_id)
+        ).scalar_one_or_none()
+        owner_id = photo.user_id if photo else None
+
         # Wipe previous detection results for this photo from this model
         session.execute(
             delete(photo_tags).where(
@@ -178,13 +184,13 @@ def detect_objects(photo_id: str):
                 best_per_label[det.label] = (det.confidence, det.bbox)
 
         for label, (confidence, bbox) in best_per_label.items():
-            # Find or create the object tag
+            # Find or create the object tag (scoped to user)
             tag = session.execute(
-                select(Tag).where(Tag.name == label, Tag.kind == 'object')
+                select(Tag).where(Tag.name == label, Tag.kind == 'object', Tag.user_id == owner_id)
             ).scalar_one_or_none()
 
             if not tag:
-                tag = Tag(name=label, kind='object', source=source_name)
+                tag = Tag(name=label, kind='object', source=source_name, user_id=owner_id)
                 session.add(tag)
                 session.flush()  # get tag.id
 
@@ -234,6 +240,12 @@ def classify_content(photo_id: str):
 
     session = _get_sync_session()
     try:
+        # Get the photo's user_id so tags inherit ownership.
+        photo = session.execute(
+            select(Photo).where(Photo.id == photo_id)
+        ).scalar_one_or_none()
+        owner_id = photo.user_id if photo else None
+
         # Wipe previous classification for this photo
         session.execute(
             delete(photo_tags).where(
@@ -242,13 +254,13 @@ def classify_content(photo_id: str):
             )
         )
 
-        # Find or create content_type tag
+        # Find or create content_type tag (scoped to user)
         tag = session.execute(
-            select(Tag).where(Tag.name == best.label, Tag.kind == 'content_type')
+            select(Tag).where(Tag.name == best.label, Tag.kind == 'content_type', Tag.user_id == owner_id)
         ).scalar_one_or_none()
 
         if not tag:
-            tag = Tag(name=best.label, kind='content_type', source=source_name)
+            tag = Tag(name=best.label, kind='content_type', source=source_name, user_id=owner_id)
             session.add(tag)
             session.flush()
 
@@ -434,11 +446,16 @@ def recluster_faces():
 
             if label not in cluster_tag_map:
                 cluster_name = f"Person {label + 1}"
+                # Inherit user_id from the representative photo.
+                rep_photo = session.execute(
+                    select(Photo.user_id).where(Photo.id == face_rows[i].photo_id)
+                ).scalar_one_or_none()
                 tag = Tag(
                     name=cluster_name,
                     kind='face_cluster',
                     source=source_name,
                     representative_photo_id=face_rows[i].photo_id,
+                    user_id=rep_photo,
                 )
                 session.add(tag)
                 session.flush()