main: bound startup sweep with context timeout

Sweep() used to run synchronously at startup without any time bound.
On a large uploads directory this can block the server for 30s or more,
keeping /healthz returning 503 while the reaper works. Add a context
timeout so startup continues if the sweep cannot finish within budget.

The timeout is set to 5 seconds: long enough to clear typical leftover
state, short enough that a stuck sweep will not meaningfully delay
health checks or startup readiness.

Change Store.Sweep to accept a context.Context and check ctx.Err() at
the top of each iteration. If the context is cancelled or times out,
Sweep returns the files deleted so far and ctx.Err().

Add TestStore_Sweep_HonorsContextCancel to ensure a cancelled context
causes Sweep to exit early instead of running to completion.

Deliberately unchanged: there is still no periodic reaper. The server
only sweeps once at startup; this change just bounds that single sweep.

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

main.go

@@ -30,6 +30,8 @@ import (
 	"spark-mcp-go/internal/uploads"
 )
 
+const startupSweepTimeout = 5 * time.Second
+
 func main() {
 	if err := run(); err != nil {
 		// run() handles its own logging; this is the last-chance report.
@@ -71,8 +73,12 @@ func run() error {
 	if err != nil {
 		return err
 	}
-	n, err := uploadStore.Sweep(cfg.UploadTTL)
-	if err != nil {
+	sweepCtx, sweepCancel := context.WithTimeout(context.Background(), startupSweepTimeout)
+	n, err := uploadStore.Sweep(sweepCtx, cfg.UploadTTL)
+	sweepCancel()
+	if errors.Is(err, context.DeadlineExceeded) {
+		logger.Warn("uploads.sweep.timeout", "deleted", n, "err", err)
+	} else if err != nil {
 		logger.Error("uploads.sweep", "err", err)
 	} else {
 		logger.Info("uploads.sweep", "deleted", n)