refactor(assets): merge AssetInfo and AssetCacheState into AssetReference

This change solves the basename collision bug by using UNIQUE(file_path) on the
unified asset_references table. Key changes:

Database:
- Migration 0005 merges asset_cache_states and asset_infos into asset_references
- AssetReference now contains: cache state fields (file_path, mtime_ns, needs_verify,
  is_missing, enrichment_level) plus info fields (name, owner_id, preview_id, etc.)
- AssetReferenceMeta replaces AssetInfoMeta
- AssetReferenceTag replaces AssetInfoTag
- UNIQUE constraint on file_path prevents duplicate entries for same file

Code:
- New unified query module: asset_reference.py (replaces asset_info.py, cache_state.py)
- Updated scanner, seeder, and services to use AssetReference
- Updated API routes to use reference_id instead of asset_info_id

Tests:
- All 175 unit tests updated and passing
- Integration tests require server environment (not run here)

Amp-Thread-ID: https://ampcode.com/threads/T-019c4fe8-9dcb-75ce-bea8-ea786343a581
Co-authored-by: Amp <amp@ampcode.com>

app/assets/api/schemas_in.py

@@ -13,7 +13,7 @@ from pydantic import (
 
 
 class UploadError(Exception):
-    """Error during upload parsing with HTTP status and code (used in HTTP layer only)."""
+    """Error during upload parsing with HTTP status and code."""
 
     def __init__(self, status: int, code: str, message: str):
         super().__init__(message)
@@ -216,14 +216,14 @@ class TagsRemove(TagsAdd):
 
 class UploadAssetSpec(BaseModel):
     """Upload Asset operation.
+
     - tags: ordered; first is root ('models'|'input'|'output');
-            if root == 'models', second must be a valid category from folder_paths.folder_names_and_paths
+            if root == 'models', second must be a valid category
     - name: display name
     - user_metadata: arbitrary JSON object (optional)
-    - hash: optional canonical 'blake3:<hex>' provided by the client for validation / fast-path
+    - hash: optional canonical 'blake3:<hex>' for validation / fast-path
 
-    Files created via this endpoint are stored on disk using the **content hash** as the filename stem
-    and the original extension is preserved when available.
+    Files are stored using the content hash as filename stem.
     """
 
     model_config = ConfigDict(extra="ignore", str_strip_whitespace=True)