feat: replace face pipeline with InsightFace, add content classifier

Face detection/recognition:
- Replace YuNet + SFace with InsightFace buffalo_l (RetinaFace + ArcFace)
- 512-d ArcFace embeddings (was 128-d SFace), migration 0006 resizes column
- Remove YOLO person-bbox workaround — RetinaFace is accurate enough
- Detection threshold 0.65 cleanly separates real faces (0.72+) from
  false positives on dogs/paintings (0.56-0.61)

Content-type classification:
- CLIP zero-shot classifier using native PyTorch text encoder + ONNX
  image encoder for high-quality text-image similarity
- Categories: photograph, screenshot, document, receipt, meme, artwork
- Writes Tag(kind=content_type) per photo via photo_tags
- Margin-based confidence: top-1 vs top-2 score difference
- New ClassifierSettings in config (enabled, min_confidence)
- Wired into vision_fanout pipeline

Tested: 6 real faces from 4 photos (zero false positives), 11/13 photos
classified (8 photograph, 2 artwork, 1 meme).

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

backend/alembic/versions/0006_face_embeddings_512d.py

@@ -0,0 +1,39 @@
+"""face_embeddings vector 128 -> 512
+
+Revision ID: 0006_face_512d
+Revises: 0005_face_embeddings
+Create Date: 2026-04-10
+
+Resize face_embeddings.vector from Vector(128) to Vector(512) for
+ArcFace embeddings (InsightFace). Drops existing data and HNSW index,
+recreates both.
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "0006_face_512d"
+down_revision: Union[str, None] = "0005_face_embeddings"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Drop index, truncate (old 128-d vectors are incompatible), resize
+    op.execute("DROP INDEX IF EXISTS ix_face_embeddings_vector_hnsw")
+    op.execute("DELETE FROM face_embeddings")
+    op.execute("ALTER TABLE face_embeddings ALTER COLUMN vector TYPE vector(512)")
+    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 INDEX IF EXISTS ix_face_embeddings_vector_hnsw")
+    op.execute("DELETE FROM face_embeddings")
+    op.execute("ALTER TABLE face_embeddings ALTER COLUMN vector TYPE vector(128)")
+    op.execute("""
+        CREATE INDEX IF NOT EXISTS ix_face_embeddings_vector_hnsw
+        ON face_embeddings USING hnsw (vector vector_cosine_ops)
+    """)