feat(assets): add background asset seeder for non-blocking startup

- Add AssetSeeder singleton class with thread management and cancellation
- Support IDLE/RUNNING/CANCELLING state machine with thread-safe access
- Emit WebSocket events for scan progress (started, progress, completed, cancelled, error)
- Update main.py to use non-blocking asset_seeder.start() at startup
- Add shutdown() call in finally block for graceful cleanup
- Update POST /api/assets/seed to return 202 Accepted, support ?wait=true
- Add GET /api/assets/seed/status and POST /api/assets/seed/cancel endpoints
- Update test helper to use ?wait=true for synchronous behavior
- Add 17 unit tests covering state transitions, cancellation, and thread safety
- Log scan configuration (models directory, input/output paths) at scan start

Amp-Thread-ID: https://ampcode.com/threads/T-019c2b45-e6e8-740a-b38b-b11daea8d094
Co-authored-by: Amp <amp@ampcode.com>

tests-unit/assets_test/helpers.py

@@ -1,13 +1,14 @@
 """Helper functions for assets integration tests."""
-import time
-
 import requests
 
 
 def trigger_sync_seed_assets(session: requests.Session, base_url: str) -> None:
-    """Force a fast sync/seed pass by calling the seed endpoint."""
-    session.post(base_url + "/api/assets/seed", json={"roots": ["models", "input", "output"]}, timeout=30)
-    time.sleep(0.2)
+    """Force a synchronous sync/seed pass by calling the seed endpoint with wait=true."""
+    session.post(
+        base_url + "/api/assets/seed?wait=true",
+        json={"roots": ["models", "input", "output"]},
+        timeout=60,
+    )
 
 
 def get_asset_filename(asset_hash: str, extension: str) -> str: