feat(assets): async two-phase scanner and background seeder

- Rewrite scanner.py with two-phase scanning architecture (fast scan + enrich)
- Add AssetSeeder for non-blocking background startup scanning
- Implement pause/resume/stop/restart controls and disable/enable for --disable-assets-autoscan
- Add non-destructive asset pruning with is_missing flag
- Wire seeder into main.py and server.py lifecycle
- Skip hidden files/directories, populate mime_type, optional blake3 hashing
- Add comprehensive seeder tests

Co-authored-by: Amp <amp@ampcode.com>
Amp-Thread-ID: https://ampcode.com/threads/T-019c9209-37af-757a-b6e4-af59b4267362

Author: luke-mino-altherr

app/assets/api/routes.py

@@ -19,7 +19,7 @@
     delete_temp_file_if_exists,
     parse_multipart_upload,
 )
-from app.assets.seeder import asset_seeder
+from app.assets.seeder import ScanInProgressError, asset_seeder
 from app.assets.services import (
     DependencyMissingError,
     HashMismatchError,
@@ -717,8 +717,9 @@ async def mark_missing_assets(request: web.Request) -> web.Response:
         200 OK with count of marked assets
         409 Conflict if a scan is currently running
     """
-    marked = asset_seeder.mark_missing_outside_prefixes()
-    if marked == 0 and asset_seeder.get_status().state.value != "IDLE":
+    try:
+        marked = asset_seeder.mark_missing_outside_prefixes()
+    except ScanInProgressError:
         return web.json_response(
             {"status": "scan_running", "marked": 0},
             status=409,