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>

app/assets/api/routes.py

@@ -15,7 +15,7 @@ from app.assets.api.schemas_in import (
     UploadError,
 )
 from app.assets.api.upload import parse_multipart_upload
-from app.assets.scanner import seed_assets as scanner_seed_assets
+from app.assets.seeder import asset_seeder
 from app.assets.services import (
     DependencyMissingError,
     HashMismatchError,
@@ -620,21 +620,78 @@ async def delete_asset_tags(request: web.Request) -> web.Response:
 
 @ROUTES.post("/api/assets/seed")
 async def seed_assets(request: web.Request) -> web.Response:
-    """Trigger asset seeding for specified roots (models, input, output)."""
+    """Trigger asset seeding for specified roots (models, input, output).
+
+    Query params:
+        wait: If "true", block until scan completes (synchronous behavior for tests)
+
+    Returns:
+        202 Accepted if scan started
+        409 Conflict if scan already running
+        200 OK with final stats if wait=true
+    """
     try:
         payload = await request.json()
         roots = payload.get("roots", ["models", "input", "output"])
     except Exception:
         roots = ["models", "input", "output"]
 
-    valid_roots = [r for r in roots if r in ("models", "input", "output")]
+    valid_roots = tuple(r for r in roots if r in ("models", "input", "output"))
     if not valid_roots:
         return _build_error_response(400, "INVALID_BODY", "No valid roots specified")
 
-    try:
-        scanner_seed_assets(tuple(valid_roots))
-    except Exception:
-        logging.exception("scanner_seed_assets failed for roots=%s", valid_roots)
-        return _build_error_response(500, "INTERNAL", "Seed operation failed")
+    wait_param = request.query.get("wait", "").lower()
+    should_wait = wait_param in ("true", "1", "yes")
 
-    return web.json_response({"seeded": valid_roots}, status=200)
+    started = asset_seeder.start(roots=valid_roots)
+    if not started:
+        return web.json_response({"status": "already_running"}, status=409)
+
+    if should_wait:
+        asset_seeder.wait()
+        status = asset_seeder.get_status()
+        return web.json_response(
+            {
+                "status": "completed",
+                "progress": {
+                    "scanned": status.progress.scanned if status.progress else 0,
+                    "total": status.progress.total if status.progress else 0,
+                    "created": status.progress.created if status.progress else 0,
+                    "skipped": status.progress.skipped if status.progress else 0,
+                },
+                "errors": status.errors,
+            },
+            status=200,
+        )
+
+    return web.json_response({"status": "started"}, status=202)
+
+
+@ROUTES.get("/api/assets/seed/status")
+async def get_seed_status(request: web.Request) -> web.Response:
+    """Get current scan status and progress."""
+    status = asset_seeder.get_status()
+    return web.json_response(
+        {
+            "state": status.state.value,
+            "progress": {
+                "scanned": status.progress.scanned,
+                "total": status.progress.total,
+                "created": status.progress.created,
+                "skipped": status.progress.skipped,
+            }
+            if status.progress
+            else None,
+            "errors": status.errors,
+        },
+        status=200,
+    )
+
+
+@ROUTES.post("/api/assets/seed/cancel")
+async def cancel_seed(request: web.Request) -> web.Response:
+    """Request cancellation of in-progress scan."""
+    cancelled = asset_seeder.cancel()
+    if cancelled:
+        return web.json_response({"status": "cancelling"}, status=200)
+    return web.json_response({"status": "idle"}, status=200)