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/hashing.py

@@ -2,10 +2,23 @@ import asyncio
 import os
 from typing import IO
 
-from blake3 import blake3
-
 DEFAULT_CHUNK = 8 * 1024 * 1024
 
+_blake3 = None
+
+
+def _get_blake3():
+    global _blake3
+    if _blake3 is None:
+        try:
+            from blake3 import blake3 as _b3
+            _blake3 = _b3
+        except ImportError:
+            raise ImportError(
+                "blake3 is required for asset hashing. Install with: pip install blake3"
+            )
+    return _blake3
+
 
 def compute_blake3_hash(
     fp: str | IO[bytes],
@@ -42,7 +55,7 @@ def _hash_file_obj(file_obj: IO, chunk_size: int = DEFAULT_CHUNK) -> str:
         if orig_pos != 0:
             file_obj.seek(0)
 
-        h = blake3()
+        h = _get_blake3()()
         while True:
             chunk = file_obj.read(chunk_size)
             if not chunk: