Read queue depths with Find, not Scan (closes #234) (#237)

Both queue-depth reads used (*gorm.DB).Scan, which swaps GORM's own trace
recorder in for the logging adapter. That recorder does not implement
gorm.ParamsFilter, so those statements logged their bound values
interpolated, bypassing the suppression added for #207.

The scan guard from #222 and the queue-depth sampler from #224 each gated
green against a next that lacked the other; both landed and next went red.

Converted to Find. The emitted SQL is identical apart from placeholders,
and both paths parse the anonymous dest schema the same way, so the
queue-depth gauges are unchanged.

internal/database/testing.go

@@ -24,11 +24,24 @@ func NewTestDatabase(db *gorm.DB) *Database {
 // NewTestWebhookDBManager creates a WebhookDBManager backed by the given
 // data directory. Intended for use in tests without the fx lifecycle.
 func NewTestWebhookDBManager(dataDir string) *WebhookDBManager {
-	return &WebhookDBManager{
-		dataDir: dataDir,
-		log: slog.New(slog.NewTextHandler(
+	return NewTestWebhookDBManagerWithLogger(
+		dataDir,
+		slog.New(slog.NewTextHandler(
 			os.Stderr,
 			&slog.HandlerOptions{Level: slog.LevelDebug},
 		)),
+	)
+}
+
+// NewTestWebhookDBManagerWithLogger is NewTestWebhookDBManager with the
+// logger supplied by the caller. The per-webhook databases this manager
+// opens hand that logger to gormlog, so a test that needs to see the SQL
+// the service emits can capture it.
+func NewTestWebhookDBManagerWithLogger(
+	dataDir string, log *slog.Logger,
+) *WebhookDBManager {
+	return &WebhookDBManager{
+		dataDir: dataDir,
+		log:     log,
 	}
 }