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/alembic/versions/0002_extend_tags_for_vision.py

@@ -0,0 +1,85 @@
+"""extend tags for vision pipeline
+
+Revision ID: 0002_extend_tags
+Revises: 0001_baseline
+Create Date: 2026-04-10
+
+Add kind, source, representative_photo_id to tags table.
+Add confidence, bbox, source to photo_tags association.
+Switch uniqueness from (name) to (name, kind).
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import JSONB
+
+revision: str = "0002_extend_tags"
+down_revision: Union[str, None] = "0001_baseline"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # ── tags table ────────────────────────────────────────────────────
+    op.execute("ALTER TABLE tags ADD COLUMN IF NOT EXISTS kind VARCHAR NOT NULL DEFAULT 'user'")
+    op.execute("ALTER TABLE tags ADD COLUMN IF NOT EXISTS source VARCHAR")
+    op.execute("ALTER TABLE tags ADD COLUMN IF NOT EXISTS representative_photo_id VARCHAR REFERENCES photos(id) ON DELETE SET NULL")
+
+    # Create index on kind for filtering
+    op.execute("CREATE INDEX IF NOT EXISTS ix_tags_kind ON tags(kind)")
+
+    # Drop old unique constraint on name (if it exists) and add (name, kind).
+    # SQLAlchemy create_all may have created either — handle both cases.
+    op.execute("""
+        DO $$
+        BEGIN
+            -- Drop the old single-column unique index/constraint if present.
+            IF EXISTS (
+                SELECT 1 FROM pg_indexes
+                WHERE tablename = 'tags' AND indexname = 'ix_tags_name'
+            ) THEN
+                DROP INDEX ix_tags_name;
+            END IF;
+
+            -- Some SQLAlchemy versions create a unique constraint directly.
+            IF EXISTS (
+                SELECT 1 FROM information_schema.table_constraints
+                WHERE table_name = 'tags' AND constraint_name = 'tags_name_key'
+            ) THEN
+                ALTER TABLE tags DROP CONSTRAINT tags_name_key;
+            END IF;
+        END $$;
+    """)
+
+    op.execute("""
+        DO $$
+        BEGIN
+            IF NOT EXISTS (
+                SELECT 1 FROM pg_constraint WHERE conname = 'uq_tags_name_kind'
+            ) THEN
+                ALTER TABLE tags ADD CONSTRAINT uq_tags_name_kind UNIQUE (name, kind);
+            END IF;
+        END $$;
+    """)
+
+    # ── photo_tags table ──────────────────────────────────────────────
+    op.execute("ALTER TABLE photo_tags ADD COLUMN IF NOT EXISTS confidence FLOAT")
+    op.execute("ALTER TABLE photo_tags ADD COLUMN IF NOT EXISTS bbox JSONB")
+    op.execute("ALTER TABLE photo_tags ADD COLUMN IF NOT EXISTS source VARCHAR")
+
+
+def downgrade() -> None:
+    # photo_tags columns
+    op.drop_column("photo_tags", "source")
+    op.drop_column("photo_tags", "bbox")
+    op.drop_column("photo_tags", "confidence")
+
+    # tags: restore old unique constraint
+    op.execute("ALTER TABLE tags DROP CONSTRAINT IF EXISTS uq_tags_name_kind")
+    op.execute("CREATE UNIQUE INDEX IF NOT EXISTS ix_tags_name ON tags(name)")
+
+    # tags columns
+    op.drop_column("tags", "representative_photo_id")
+    op.drop_column("tags", "source")
+    op.drop_column("tags", "kind")