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)

py/services/base_model_service.py

@@ -446,20 +446,31 @@ class BaseModelService(ABC):
     async def _apply_hash_filters(
         self, data: List[Dict], hash_filters: Dict
     ) -> List[Dict]:
-        """Apply hash-based filtering"""
+        """Apply hash-based filtering (SHA256 and AutoV3)."""
+
+        def matches_hash_set(item: Dict, hash_set: set) -> bool:
+            """Check whether an item matches any hash in the set.
+
+            Compares the item's ``sha256`` field and its non-empty ``autov3``
+            field, both case-insensitively.
+            """
+            if item.get("sha256", "").lower() in hash_set:
+                return True
+            autov3 = item.get("autov3", "")
+            return bool(autov3) and autov3.lower() in hash_set
+
         single_hash = hash_filters.get("single_hash")
         multiple_hashes = hash_filters.get("multiple_hashes")
 
         if single_hash:
-            # Filter by single hash
-            single_hash = single_hash.lower()
+            # Filter by single hash (SHA256 or AutoV3)
             return [
-                item for item in data if item.get("sha256", "").lower() == single_hash
+                item for item in data if matches_hash_set(item, {single_hash.lower()})
             ]
         elif multiple_hashes:
-            # Filter by multiple hashes
-            hash_set = set(hash.lower() for hash in multiple_hashes)
-            return [item for item in data if item.get("sha256", "").lower() in hash_set]
+            # Filter by multiple hashes (SHA256 or AutoV3)
+            hash_set = {hash.lower() for hash in multiple_hashes}
+            return [item for item in data if matches_hash_set(item, hash_set)]
 
         return data