feat: add face detection, recognition, and clustering

- Create face_embeddings table with pgvector Vector(128) + HNSW index
- Implement extract_faces task (YuNet detection + SFace recognition)
- Implement recluster_faces task (DBSCAN clustering → Tag(kind=face_cluster))
- Clusters are named "Person N" and get representative_photo_id
- cluster_id FK → tags.id, SET NULL on delete for merge/rename support

Migration 0005 creates the face_embeddings table.

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

backend/alembic/versions/0005_face_embeddings.py

@@ -0,0 +1,41 @@
+"""face_embeddings table
+
+Revision ID: 0005_face_embeddings
+Revises: 0004_ocr_fts
+Create Date: 2026-04-10
+
+Create face_embeddings table with pgvector Vector(128) for SFace
+recognition embeddings and HNSW index.
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "0005_face_embeddings"
+down_revision: Union[str, None] = "0004_ocr_fts"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.execute("""
+        CREATE TABLE IF NOT EXISTS face_embeddings (
+            id VARCHAR PRIMARY KEY,
+            photo_id VARCHAR NOT NULL REFERENCES photos(id) ON DELETE CASCADE,
+            bbox JSONB,
+            vector vector(128),
+            cluster_id VARCHAR REFERENCES tags(id) ON DELETE SET NULL,
+            quality FLOAT,
+            created_at TIMESTAMPTZ DEFAULT now()
+        )
+    """)
+    op.execute("CREATE INDEX IF NOT EXISTS ix_face_embeddings_photo_id ON face_embeddings(photo_id)")
+    op.execute("CREATE INDEX IF NOT EXISTS ix_face_embeddings_cluster_id ON face_embeddings(cluster_id)")
+    op.execute("""
+        CREATE INDEX IF NOT EXISTS ix_face_embeddings_vector_hnsw
+        ON face_embeddings USING hnsw (vector vector_cosine_ops)
+    """)
+
+
+def downgrade() -> None:
+    op.execute("DROP TABLE IF EXISTS face_embeddings")