refactor(handlers): migrate remaining backend handlers to clock.Now() and transaction patterns

Apply clock.Now() migration, transaction wrapping, and minor refactors across admin, scheduling, today, user, auth handler, notifications, webhooks, services, portfolio, ratelimit, testutils, and main.go.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/admin/bookings_extra_test.go

@@ -8,6 +8,7 @@ import (
 	"testing"
 	"time"
 
+	"crussell/clock"
 	"crussell/testutils"
 	"crussell/handlers/bookings"
 	"crussell/testutils/fixtures"
@@ -116,7 +117,7 @@ func TestAdminRescheduleBooking(t *testing.T) {
 		t.Fatalf("failed to create test service: %v", err)
 	}
 
-	startTime := time.Now().Add(48 * time.Hour).Truncate(time.Second)
+	startTime := clock.Now().Add(48 * time.Hour).Truncate(time.Second)
 	bookingID, err := fixtures.CreateTestBookingAtTime(tx, userID, serviceID, startTime)
 	if err != nil {
 		t.Fatalf("failed to create test booking: %v", err)