Delete dead test helpers and rename vestigial test paths

- Remove seed_helpers.go (SeedTestMessage, SeedTestMessages,
  SeedTestCursor — all unused, channel/message features gutted)
- Rename channel- prefixed test paths to test- in storage tests

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/storage/pvc_test.go

@@ -24,7 +24,7 @@ func TestPVC_PutGetRoundTrip(t *testing.T) {
 	ctx := context.Background()
 
 	data := []byte("hello, storage world")
-	key := "files/channel-1/att-1_test.txt"
+	key := "files/test-1/att-1_test.txt"
 
 	// Put
 	err := s.Put(ctx, key, bytes.NewReader(data), int64(len(data)), "text/plain")
@@ -129,18 +129,18 @@ func TestPVC_DeletePrefix(t *testing.T) {
 	ctx := context.Background()
 
 	// Create several files under a channel prefix
-	prefix := "files/channel-abc/"
+	prefix := "files/test-abc/"
 	for _, name := range []string{"a.txt", "b.png", "c.pdf"} {
 		key := prefix + name
 		_ = s.Put(ctx, key, bytes.NewReader([]byte("x")), 1, "text/plain")
 	}
 
 	// Also create a file in a different channel
-	other := "files/channel-other/keep.txt"
+	other := "files/test-other/keep.txt"
 	_ = s.Put(ctx, other, bytes.NewReader([]byte("y")), 1, "text/plain")
 
 	// Delete the channel prefix
-	err := s.DeletePrefix(ctx, "files/channel-abc")
+	err := s.DeletePrefix(ctx, "files/test-abc")
 	if err != nil {
 		t.Fatalf("DeletePrefix: %v", err)
 	}