Keep temp-directory assets visible while their files exist

Assets written to the temp directory were flagged as missing and dropped
from GET /api/assets, even with the file sitting on disk. One list of
directories was answering two different questions -- where the scanner
looks for new files, and which files ComfyUI considers its own -- and
temp belongs only in the second, so every temp reference was disowned by
the prune that runs at startup and on POST /api/assets/prune.

Ownership now covers temp. Discovery still does not: the temp directory
is wiped before the scan runs, and assets written there are already
registered with a hash, mime type and dimensions, so walking it would
find nothing. Temp references are instead reconciled against the
filesystem directly, so a temp file that really is gone is still retired
rather than lingering as a broken entry.

get_prefixes_for_root becomes get_scan_prefixes_for_root so the two
questions are told apart by name rather than by comment.

tests/test_asset_seeder.py

@@ -162,7 +162,7 @@ class TestPendingEnrichDrain:
     """Verify that _run_scan drains _pending_enrich via start_enrich."""
 
     @patch("app.assets.seeder.dependencies_available", return_value=True)
-    @patch("app.assets.seeder.get_all_known_prefixes", return_value=[])
+    @patch("app.assets.seeder.get_owned_prefixes", return_value=[])
     @patch("app.assets.seeder.sync_root_safely", return_value=set())
     @patch("app.assets.seeder.collect_paths_for_roots", return_value=[])
     @patch("app.assets.seeder.build_asset_specs", return_value=([], {}, 0))
@@ -187,7 +187,7 @@ class TestPendingEnrichDrain:
         assert seeder._pending_enrich is None
 
     @patch("app.assets.seeder.dependencies_available", return_value=True)
-    @patch("app.assets.seeder.get_all_known_prefixes", return_value=[])
+    @patch("app.assets.seeder.get_owned_prefixes", return_value=[])
     @patch("app.assets.seeder.sync_root_safely", return_value=set())
     @patch("app.assets.seeder.collect_paths_for_roots", return_value=[])
     @patch("app.assets.seeder.build_asset_specs", return_value=([], {}, 0))
@@ -206,7 +206,7 @@ class TestPendingEnrichDrain:
         assert seeder._pending_enrich is None
 
     @patch("app.assets.seeder.dependencies_available", return_value=True)
-    @patch("app.assets.seeder.get_all_known_prefixes", return_value=[])
+    @patch("app.assets.seeder.get_owned_prefixes", return_value=[])
     @patch("app.assets.seeder.sync_root_safely", return_value=set())
     @patch("app.assets.seeder.collect_paths_for_roots", return_value=[])
     @patch("app.assets.seeder.build_asset_specs", return_value=([], {}, 0))