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/base_model_service.py

@@ -5,7 +5,7 @@ import logging
 import os
 import time
 
-from ..utils.constants import VALID_LORA_TYPES
+from ..utils.constants import VALID_LORA_SUB_TYPES, VALID_CHECKPOINT_SUB_TYPES
 from ..utils.models import BaseModelMetadata
 from ..utils.metadata_manager import MetadataManager
 from ..utils.usage_stats import UsageStats
@@ -15,8 +15,8 @@ from .model_query import (
     ModelFilterSet,
     SearchStrategy,
     SettingsProvider,
-    normalize_civitai_model_type,
-    resolve_civitai_model_type,
+    normalize_sub_type,
+    resolve_sub_type,
 )
 from .settings_manager import get_settings_manager
 
@@ -568,16 +568,21 @@ class BaseModelService(ABC):
         return await self.scanner.get_base_models(limit)
 
     async def get_model_types(self, limit: int = 20) -> List[Dict[str, Any]]:
-        """Get counts of normalized CivitAI model types present in the cache."""
+        """Get counts of sub-types present in the cache."""
         cache = await self.scanner.get_cached_data()
 
         type_counts: Dict[str, int] = {}
         for entry in cache.raw_data:
-            normalized_type = normalize_civitai_model_type(
-                resolve_civitai_model_type(entry)
-            )
-            if not normalized_type or normalized_type not in VALID_LORA_TYPES:
+            normalized_type = normalize_sub_type(resolve_sub_type(entry))
+            if not normalized_type:
                 continue
+            
+            # Filter by valid sub-types based on scanner type
+            if self.model_type == "lora" and normalized_type not in VALID_LORA_SUB_TYPES:
+                continue
+            if self.model_type == "checkpoint" and normalized_type not in VALID_CHECKPOINT_SUB_TYPES:
+                continue
+            
             type_counts[normalized_type] = type_counts.get(normalized_type, 0) + 1
 
         sorted_types = sorted(