feat: add thumbnail metadata and download endpoint

- Extend `BoxFile` with thumbnail path/status fields and internal URL
- Populate `ThumbnailURL` when a thumbnail path is present during decoration
- Add `/box/:id/thumbnails/:file_id` route and handler to serve JPEG thumbnails
- Introduce thumbnail status constants to standardize processing state reportingfeat: add thumbnail metadata and download endpoint

- Extend `BoxFile` with thumbnail path/status fields and internal URL
- Populate `ThumbnailURL` when a thumbnail path is present during decoration
- Add `/box/:id/thumbnails/:file_id` route and handler to serve JPEG thumbnails
- Introduce thumbnail status constants to standardize processing state reporting

lib/boxstore/store_test.go

@@ -0,0 +1,44 @@
+package boxstore
+
+import (
+	"testing"
+	"time"
+
+	"warpbox/lib/models"
+)
+
+func TestStartRetentionWaitsForEveryFileToFinish(t *testing.T) {
+	manifest := models.BoxManifest{
+		RetentionSecs: 10,
+		Files: []models.BoxFile{
+			{ID: "one", Status: models.FileStatusReady},
+			{ID: "two", Status: models.FileStatusWork},
+		},
+	}
+
+	startRetentionIfTerminalUnlocked(&manifest)
+
+	if !manifest.ExpiresAt.IsZero() {
+		t.Fatalf("expected retention to stay unset while a file is still uploading, got %s", manifest.ExpiresAt)
+	}
+}
+
+func TestStartRetentionBeginsWhenEveryFileIsTerminal(t *testing.T) {
+	manifest := models.BoxManifest{
+		RetentionSecs: 10,
+		Files: []models.BoxFile{
+			{ID: "one", Status: models.FileStatusReady},
+			{ID: "two", Status: models.FileStatusFailed},
+		},
+	}
+	before := time.Now().UTC()
+
+	startRetentionIfTerminalUnlocked(&manifest)
+
+	if manifest.ExpiresAt.IsZero() {
+		t.Fatal("expected retention to start once every file is complete or failed")
+	}
+	if manifest.ExpiresAt.Before(before.Add(9 * time.Second)) {
+		t.Fatalf("expected retention to start from completion time, got %s", manifest.ExpiresAt)
+	}
+}