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/testutils/testdb/testdb.go

@@ -75,6 +75,7 @@ func CreateTestDatabase(dbName string) *pgxpool.Pool {
 	}
 	poolCfg.MaxConns = 16
 	poolCfg.ConnConfig.DefaultQueryExecMode = pgx.QueryExecModeSimpleProtocol
+	poolCfg.ConnConfig.RuntimeParams["timezone"] = "UTC"
 	pool, err := pgxpool.NewWithConfig(ctx, poolCfg)
 	if err != nil {
 		log.Fatalf("testdb: failed to connect to %s: %v", dbName, err)
@@ -153,7 +154,12 @@ func Pool(t *testing.T) *pgxpool.Pool {
 	}
 
 	ctx := context.Background()
-	pool, err := pgxpool.New(ctx, dsn)
+	poolCfg, err := pgxpool.ParseConfig(dsn)
+	if err != nil {
+		t.Fatalf("Failed to parse config: %v", err)
+	}
+	poolCfg.ConnConfig.RuntimeParams["timezone"] = "UTC"
+	pool, err := pgxpool.NewWithConfig(ctx, poolCfg)
 	if err != nil {
 		t.Fatalf("Failed to connect to test database: %v", err)
 	}
@@ -171,7 +177,12 @@ func NewPool(dsn string) (*pgxpool.Pool, error) {
 	}
 
 	ctx := context.Background()
-	pool, err := pgxpool.New(ctx, dsn)
+	poolCfg, err := pgxpool.ParseConfig(dsn)
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse config: %w", err)
+	}
+	poolCfg.ConnConfig.RuntimeParams["timezone"] = "UTC"
+	pool, err := pgxpool.NewWithConfig(ctx, poolCfg)
 	if err != nil {
 		return nil, fmt.Errorf("failed to create pool: %w", err)
 	}