Clean up imports, remove section banners, and fix seed endpoint with autoscan disabled

Move inline/lazy imports to top-level now that blake3 is a main dependency and
circular-import concerns are resolved. Remove decorative section-header comments
in asset_reference queries. Allow the seed API endpoint to temporarily re-enable
the seeder when --disable-assets-autoscan is active so manual triggers still work.
Add retry-on-409 logic in test helpers and remove try/finally around prompt
execution in favour of sequential pause/resume.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/assets/services/hashing.py

@@ -2,23 +2,10 @@ 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],
@@ -55,7 +42,7 @@ def _hash_file_obj(file_obj: IO, chunk_size: int = DEFAULT_CHUNK) -> str:
         if orig_pos != 0:
             file_obj.seek(0)
 
-        h = _get_blake3()()
+        h = blake3()
         while True:
             chunk = file_obj.read(chunk_size)
             if not chunk: