refactor: unify model_type semantics by introducing sub_type field

This commit resolves the semantic confusion around the model_type field by
clearly distinguishing between:
- scanner_type: architecture-level (lora/checkpoint/embedding)
- sub_type: business-level subtype (lora/locon/dora/checkpoint/diffusion_model/embedding)

Backend Changes:
- Rename model_type to sub_type in CheckpointMetadata and EmbeddingMetadata
- Add resolve_sub_type() and normalize_sub_type() in model_query.py
- Update checkpoint_scanner to use _resolve_sub_type()
- Update service format_response to include both sub_type and model_type
- Add VALID_*_SUB_TYPES constants with backward compatible aliases

Frontend Changes:
- Add MODEL_SUBTYPE_DISPLAY_NAMES constants
- Keep MODEL_TYPE_DISPLAY_NAMES as backward compatible alias

Testing:
- Add 43 new tests covering sub_type resolution and API response

Documentation:
- Add refactoring todo document to docs/technical/

BREAKING CHANGE: None - full backward compatibility maintained

py/services/checkpoint_scanner.py

@@ -21,7 +21,8 @@ class CheckpointScanner(ModelScanner):
             hash_index=ModelHashIndex()
         )
 
-    def _resolve_model_type(self, root_path: Optional[str]) -> Optional[str]:
+    def _resolve_sub_type(self, root_path: Optional[str]) -> Optional[str]:
+        """Resolve the sub-type based on the root path."""
         if not root_path:
             return None
 
@@ -34,18 +35,28 @@ class CheckpointScanner(ModelScanner):
         return None
 
     def adjust_metadata(self, metadata, file_path, root_path):
-        if hasattr(metadata, "model_type"):
-            model_type = self._resolve_model_type(root_path)
-            if model_type:
-                metadata.model_type = model_type
+        """Adjust metadata during scanning to set sub_type."""
+        # Support both old 'model_type' and new 'sub_type' for backward compatibility
+        if hasattr(metadata, "sub_type"):
+            sub_type = self._resolve_sub_type(root_path)
+            if sub_type:
+                metadata.sub_type = sub_type
+        elif hasattr(metadata, "model_type"):
+            # Backward compatibility: fallback to model_type if sub_type not available
+            sub_type = self._resolve_sub_type(root_path)
+            if sub_type:
+                metadata.model_type = sub_type
         return metadata
 
     def adjust_cached_entry(self, entry: Dict[str, Any]) -> Dict[str, Any]:
-        model_type = self._resolve_model_type(
+        """Adjust entries loaded from the persisted cache to ensure sub_type is set."""
+        sub_type = self._resolve_sub_type(
             self._find_root_for_file(entry.get("file_path"))
         )
-        if model_type:
-            entry["model_type"] = model_type
+        if sub_type:
+            entry["sub_type"] = sub_type
+            # Also set model_type for backward compatibility during transition
+            entry["model_type"] = sub_type
         return entry
 
     def get_model_roots(self) -> List[str]: