feat: lazy hash calculation for checkpoints

Checkpoints are typically large (10GB+). This change delays SHA256
hash calculation until metadata fetch from Civitai is requested,
significantly improving initial scan performance.

- Add hash_status field to BaseModelMetadata
- CheckpointScanner skips hash during initial scan
- On-demand hash calculation during Civitai fetch
- Background bulk hash calculation support

py/services/model_scanner.py

@@ -282,6 +282,11 @@ class ModelScanner:
         sub_type = get_value('sub_type', None)
         if sub_type:
             entry['sub_type'] = sub_type
+        
+        # Handle hash_status for lazy hash calculation (checkpoints)
+        hash_status = get_value('hash_status', 'completed')
+        if hash_status:
+            entry['hash_status'] = hash_status
 
         return entry