Add optional blake3 hashing during asset scanning

- Make blake3 import lazy in hashing.py (only imported when needed)
- Add compute_hashes parameter to AssetSeeder.start(), build_asset_specs(), and seed_assets()
- Fix missing tag clearing: include is_missing states in sync when update_missing_tags=True
- Clear is_missing flag on cache states when files are restored with matching mtime/size
- Fix validation error serialization in routes.py (use json.loads(ve.json()))

Amp-Thread-ID: https://ampcode.com/threads/T-019c3614-56d4-74a8-a717-19922d6dbbee
Co-authored-by: Amp <amp@ampcode.com>

app/assets/services/file_utils.py

@@ -23,15 +23,16 @@ def verify_file_unchanged(
 
     Returns True if the file's mtime and size match the database values.
     Returns False if mtime_db is None or values don't match.
+
+    size_db=None means don't check size; 0 is a valid recorded size.
     """
     if mtime_db is None:
         return False
     actual_mtime_ns = get_mtime_ns(stat_result)
     if int(mtime_db) != int(actual_mtime_ns):
         return False
-    sz = int(size_db or 0)
-    if sz > 0:
-        return int(stat_result.st_size) == sz
+    if size_db is not None:
+        return int(stat_result.st_size) == int(size_db)
     return True