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/routers/tags.py

@@ -1,10 +1,15 @@
 """
-Tags API router
+Tags API router.
+
+Unified across user tags, ML-detected objects, and face clusters via
+the `kind` query parameter. Default behaviour (no kind filter) returns
+all tags — the frontend's "Hide auto-generated tags" toggle filters
+client-side or passes `kind=user`.
 """
 from typing import Optional
-from fastapi import APIRouter, Depends, HTTPException
+from fastapi import APIRouter, Depends, HTTPException, Query
 from pydantic import BaseModel
-from sqlalchemy import select, func, insert, delete
+from sqlalchemy import select, func, update
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.database import get_db
@@ -19,6 +24,7 @@ router = APIRouter()
 class TagCreate(BaseModel):
     name: str
     color: Optional[str] = None
+    kind: str = "user"
 
 
 class TagUpdate(BaseModel):
@@ -26,11 +32,18 @@ class TagUpdate(BaseModel):
     color: Optional[str] = None
 
 
+class TagMerge(BaseModel):
+    target_id: str  # tag to merge INTO
+
+
 # ── Endpoints ─────────────────────────────────────────────────────────────
 
 @router.get("")
-async def list_tags(db: AsyncSession = Depends(get_db)):
-    """List all tags with their photo counts."""
+async def list_tags(
+    kind: Optional[str] = Query(None, description="Filter by kind: user, object, scene, face_cluster"),
+    db: AsyncSession = Depends(get_db),
+):
+    """List all tags with their photo counts, optionally filtered by kind."""
     count_subq = (
         select(
             photo_tags.c.tag_id,
@@ -42,8 +55,11 @@ async def list_tags(db: AsyncSession = Depends(get_db)):
     stmt = (
         select(Tag, count_subq.c.photo_count)
         .outerjoin(count_subq, Tag.id == count_subq.c.tag_id)
-        .order_by(Tag.name.asc())
     )
+    if kind:
+        stmt = stmt.where(Tag.kind == kind)
+    stmt = stmt.order_by(Tag.name.asc())
+
     result = await db.execute(stmt)
     rows = result.all()
 
@@ -52,6 +68,9 @@ async def list_tags(db: AsyncSession = Depends(get_db)):
             "id": tag.id,
             "name": tag.name,
             "color": tag.color,
+            "kind": tag.kind,
+            "source": tag.source,
+            "representative_photo_id": tag.representative_photo_id,
             "photo_count": int(count or 0),
         }
         for tag, count in rows
@@ -60,30 +79,37 @@ async def list_tags(db: AsyncSession = Depends(get_db)):
 
 @router.post("", status_code=201)
 async def create_tag(body: TagCreate, db: AsyncSession = Depends(get_db)):
-    """Create a new tag. Names are unique — re-creating an existing name
-    returns the existing row instead of erroring (idempotent for the
-    autocomplete UI flow)."""
+    """Create a new tag. The (name, kind) pair is unique — re-creating an
+    existing pair returns the existing row (idempotent for autocomplete)."""
     name = (body.name or "").strip()
     if not name:
         raise HTTPException(status_code=400, detail="Tag name is required")
 
-    existing = await db.execute(select(Tag).where(Tag.name == name))
+    existing = await db.execute(
+        select(Tag).where(Tag.name == name, Tag.kind == body.kind)
+    )
     found = existing.scalar_one_or_none()
     if found:
-        return {"id": found.id, "name": found.name, "color": found.color, "photo_count": 0}
+        return {
+            "id": found.id, "name": found.name, "color": found.color,
+            "kind": found.kind, "photo_count": 0,
+        }
 
-    tag = Tag(name=name, color=body.color)
+    tag = Tag(name=name, color=body.color, kind=body.kind)
     db.add(tag)
     await db.commit()
     await db.refresh(tag)
-    return {"id": tag.id, "name": tag.name, "color": tag.color, "photo_count": 0}
+    return {
+        "id": tag.id, "name": tag.name, "color": tag.color,
+        "kind": tag.kind, "photo_count": 0,
+    }
 
 
 @router.patch("/{tag_id}")
 async def update_tag(
     tag_id: str, body: TagUpdate, db: AsyncSession = Depends(get_db)
 ):
-    """Rename or recolor a tag."""
+    """Rename or recolor a tag (works for any kind — user, object, face_cluster)."""
     result = await db.execute(select(Tag).where(Tag.id == tag_id))
     tag = result.scalar_one_or_none()
     if not tag:
@@ -99,7 +125,52 @@ async def update_tag(
 
     await db.commit()
     await db.refresh(tag)
-    return {"id": tag.id, "name": tag.name, "color": tag.color}
+    return {"id": tag.id, "name": tag.name, "color": tag.color, "kind": tag.kind}
+
+
+@router.post("/{tag_id}/merge")
+async def merge_tag(
+    tag_id: str, body: TagMerge, db: AsyncSession = Depends(get_db)
+):
+    """Merge tag_id INTO target_id. All photo associations from the source
+    tag are moved to the target, then the source tag is deleted.
+
+    Useful for merging auto-detected face clusters (e.g. "Person 3" → "Alice")
+    or merging duplicate object labels."""
+    if tag_id == body.target_id:
+        raise HTTPException(status_code=400, detail="Cannot merge a tag into itself")
+
+    source = (await db.execute(select(Tag).where(Tag.id == tag_id))).scalar_one_or_none()
+    target = (await db.execute(select(Tag).where(Tag.id == body.target_id))).scalar_one_or_none()
+    if not source:
+        raise HTTPException(status_code=404, detail="Source tag not found")
+    if not target:
+        raise HTTPException(status_code=404, detail="Target tag not found")
+
+    # Move photo associations: update tag_id from source → target.
+    # Skip any that would violate the PK (photo already tagged with target).
+    existing_target_photos = select(photo_tags.c.photo_id).where(
+        photo_tags.c.tag_id == body.target_id
+    )
+    await db.execute(
+        update(photo_tags)
+        .where(
+            photo_tags.c.tag_id == tag_id,
+            photo_tags.c.photo_id.notin_(existing_target_photos),
+        )
+        .values(tag_id=body.target_id)
+    )
+    # Delete remaining source associations (duplicates that couldn't move)
+    from sqlalchemy import delete as sa_delete
+    await db.execute(
+        sa_delete(photo_tags).where(photo_tags.c.tag_id == tag_id)
+    )
+
+    # Delete source tag
+    await db.delete(source)
+    await db.commit()
+
+    return {"merged_into": target.id, "target_name": target.name}
 
 
 @router.delete("/{tag_id}", status_code=204)