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

static/js/utils/constants.js

@@ -57,12 +57,22 @@ export const BASE_MODELS = {
     UNKNOWN: "Other"
 };
 
-export const MODEL_TYPE_DISPLAY_NAMES = {
+// Model sub-type display names (new canonical field: sub_type)
+export const MODEL_SUBTYPE_DISPLAY_NAMES = {
+    // LoRA sub-types
     lora: "LoRA",
     locon: "LyCORIS",
     dora: "DoRA",
+    // Checkpoint sub-types
+    checkpoint: "Checkpoint",
+    diffusion_model: "Diffusion Model",
+    // Embedding sub-types
+    embedding: "Embedding",
 };
 
+// Backward compatibility alias
+export const MODEL_TYPE_DISPLAY_NAMES = MODEL_SUBTYPE_DISPLAY_NAMES;
+
 export const BASE_MODEL_ABBREVIATIONS = {
     // Stable Diffusion 1.x models
     [BASE_MODELS.SD_1_4]: 'SD1',