feat(metadata): add CivitAI AutoV3 hash support across all storage layers

- Three-state autov3 field (not-checked / checked-unavailable / 12-hex value)
  in .metadata.json sidecars, in-memory ModelHashIndex, and SQLite
  (models.autov3 column + autov3_index table) with column-presence migration
- Background self-terminating backfill for legacy rows: per-model-type
  concurrency guard, executor-offloaded I/O, Civitai-first resolution
  (SHA256-matched version file) falling back to the embedded safetensors
  header hash
- Civitai-first propagation on metadata refresh, scan, and download paths;
  reject the empty-string SHA256 placeholder and strip OneTrainer 0x prefix
- List API hash filters and hash index lookups accept 12-char AutoV3
- Cap safetensors header reads at 64 MiB to prevent crafted-file allocation
- Prevent stale AutoV3 mappings on file replacement while preserving them on
  same-file re-registration (lazy-hash completion)

tests/utils/test_metadata_defaults.py

@@ -1,10 +1,18 @@
 import json
+import struct
 import pytest
 
 from py.utils.metadata_manager import MetadataManager
 from py.utils.models import BaseModelMetadata
 
 
+def _write_safetensors(path, metadata, payload=b"payload-bytes"):
+    """Write a minimal real safetensors file: 8-byte little-endian header
+    length, a JSON header containing ``__metadata__``, then arbitrary payload."""
+    header = json.dumps({"__metadata__": metadata}).encode("utf-8")
+    path.write_bytes(struct.pack("<Q", len(header)) + header + payload)
+
+
 @pytest.mark.asyncio
 async def test_base_model_metadata_sets_empty_civitai_dict():
     metadata = BaseModelMetadata(
@@ -35,3 +43,35 @@ async def test_create_default_metadata_uses_empty_civitai(tmp_path):
     payload = json.loads(metadata_path.read_text(encoding="utf-8"))
 
     assert payload.get("civitai") == {}
+
+
+@pytest.mark.asyncio
+async def test_create_default_metadata_writes_autov3_from_safetensors(tmp_path):
+    model_path = tmp_path / "example.safetensors"
+    _write_safetensors(model_path, {"sshs_model_hash": "abcdef1234567890abcdef"})
+
+    metadata = await MetadataManager.create_default_metadata(str(model_path))
+
+    assert metadata is not None
+    assert metadata.autov3 == "abcdef123456"
+
+    metadata_path = model_path.with_suffix(".metadata.json")
+    payload = json.loads(metadata_path.read_text(encoding="utf-8"))
+
+    assert payload.get("autov3") == "abcdef123456"
+
+
+@pytest.mark.asyncio
+async def test_create_default_metadata_writes_autov3_null_for_non_safetensors(tmp_path):
+    model_path = tmp_path / "example.safetensors"
+    model_path.write_bytes(b"stub")
+
+    metadata = await MetadataManager.create_default_metadata(str(model_path))
+
+    assert metadata is not None
+    assert metadata.autov3 == ""
+
+    metadata_path = model_path.with_suffix(".metadata.json")
+    payload = json.loads(metadata_path.read_text(encoding="utf-8"))
+
+    assert payload.get("autov3") is None