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/cache_entry_validator.py

@@ -59,6 +59,7 @@ class CacheEntryValidator:
         'notes': ('', False),
         'usage_tips': ('', False),
         'hash_status': ('completed', False),
+        'autov3': (None, False),
     }
 
     @classmethod
@@ -119,8 +120,13 @@ class CacheEntryValidator:
                 if is_required:
                     errors.append(f"Required field '{field_name}' is missing or None")
                 if auto_repair:
-                    working_entry[field_name] = cls._get_default_copy(default_value)
-                    repaired = True
+                    # A missing optional field whose default is None is already
+                    # semantically equal to its default (e.g. autov3: absent
+                    # means "not checked") — writing None back is a no-op, not
+                    # a repair.
+                    if default_value is not None:
+                        working_entry[field_name] = cls._get_default_copy(default_value)
+                        repaired = True
                 continue
 
             # Validate field type and value
@@ -175,6 +181,15 @@ class CacheEntryValidator:
                     # that invalidates the entry, but we also don't mark it repaired.
                     pass
 
+        # Normalize autov3 to lowercase if needed (optional field, never stripped).
+        autov3 = working_entry.get('autov3')
+        if isinstance(autov3, str) and autov3:
+            normalized_autov3 = autov3.lower()
+            if normalized_autov3 != autov3:
+                if auto_repair:
+                    working_entry['autov3'] = normalized_autov3
+                    repaired = True
+
         # Determine if entry is valid
         # Entry is valid if no critical required field errors remain after repair
         # Critical fields are file_path and sha256
@@ -242,6 +257,19 @@ class CacheEntryValidator:
         """
         expected_type = type(default_value)
 
+        # Special case: autov3 is optional with a three-state contract.
+        # None = not checked, "" = checked but unavailable, otherwise a
+        # 12-character hex string (case-insensitive here; normalized to
+        # lowercase separately).
+        if field_name == 'autov3':
+            if value is None or value == "":
+                return None
+            if not isinstance(value, str):
+                return f"Field 'autov3' should be string or None, got {type(value).__name__}"
+            if len(value) != 12 or any(c not in '0123456789abcdefABCDEF' for c in value):
+                return "Field 'autov3' should be a 12-character hex string"
+            return None
+
         # Special handling for numeric types
         if expected_type == int:
             if not isinstance(value, (int, float)):