feat: extend Tag model for unified ML tagging

Unify object detections, scene labels, and face clusters with user tags
via new columns on the existing Tag model:
- kind (user|object|scene|face_cluster), source, representative_photo_id
- photo_tags gains confidence, bbox (JSONB), source per-association
- Uniqueness moves from (name) to (name, kind) so ML labels coexist
  with user tags without collision

Add Alembic migration 0002 with defensive IF NOT EXISTS guards.

Update tags router: kind filter on GET, merge endpoint for combining
auto-detected clusters/objects, include kind/source in list response.

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

backend/app/models/tags.py

@@ -1,8 +1,14 @@
 """
-Tag model definitions
+Tag model definitions.
+
+Tags are unified across user-created tags, ML-detected objects, scene
+labels, and face clusters via the `kind` column. The `photo_tags`
+association carries per-photo ML metadata (confidence, bounding box,
+source model).
 """
-from sqlalchemy import Column, String, ForeignKey, Table, Index
+from sqlalchemy import Column, String, Float, ForeignKey, Table, Index, UniqueConstraint
 from sqlalchemy.orm import relationship
+from sqlalchemy.dialects.postgresql import JSONB
 import uuid
 
 from app.database import Base
@@ -13,20 +19,41 @@ photo_tags = Table(
     Base.metadata,
     Column('photo_id', String, ForeignKey('photos.id', ondelete='CASCADE'), primary_key=True),
     Column('tag_id', String, ForeignKey('tags.id', ondelete='CASCADE'), primary_key=True),
+    # ML metadata — null for user-applied tags
+    Column('confidence', Float, nullable=True),
+    Column('bbox', JSONB, nullable=True),       # [x1, y1, x2, y2] normalized 0-1
+    Column('source', String, nullable=True),     # e.g. "vision:yolov8n", "vision:sface"
     Index('ix_photo_tags_photo_id', 'photo_id'),
     Index('ix_photo_tags_tag_id', 'tag_id'),
 )
 
 class Tag(Base):
     __tablename__ = 'tags'
-    
+    __table_args__ = (
+        UniqueConstraint('name', 'kind', name='uq_tags_name_kind'),
+    )
+
     id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
-    name = Column(String, unique=True, nullable=False, index=True)
+    name = Column(String, nullable=False, index=True)
     color = Column(String)  # Hex color code for UI display
-    
+
+    # Tag classification
+    kind = Column(String, nullable=False, default='user', index=True)
+    # kind values: 'user' | 'object' | 'scene' | 'face_cluster'
+
+    # Which model produced this tag (null for user-created)
+    source = Column(String, nullable=True)
+    # e.g. "vision:yolov8n", "vision:sface", null
+
+    # For face clusters: the photo used as the cluster representative thumbnail
+    representative_photo_id = Column(
+        String, ForeignKey('photos.id', ondelete='SET NULL'), nullable=True
+    )
+
     # Relationships
     photos = relationship("Photo", secondary=photo_tags, backref="tags")
 
+
 class PhotoTag:
     """Helper class for photo-tag associations (not a table model)"""
-    pass
+    pass