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/main.go

@@ -121,7 +121,6 @@ func healthCheckHandler(w http.ResponseWriter, r *http.Request) {
 		services["s3_storage"] = "not_configured"
 	}
 
-	w.Header().Set("Content-Type", "application/json")
 	if status == "degraded" {
 		w.WriteHeader(http.StatusServiceUnavailable)
 	} else {
@@ -180,6 +179,7 @@ func main() {
 
 	// All API routes grouped under /api for clarity
 	r.Route("/api", func(r chi.Router) {
+		r.Use(mw.JsonContentType)
 
 		// Public read-only (but check auth context if present for eligibility)
 		r.Group(func(r chi.Router) {