refactor: use 'c' alias for constants package

- Update all imports from 'constants' to 'c' for brevity
- Replace all 'constants.' references with 'c.'
- Fix remaining hardcoded content-type headers in httputil
- Fix remaining hardcoded User-Agent and Accept headers
- Rename CSRF receiver from 'c' to 'csrf' to avoid conflict
- Add ContentTypePlainSimple constant for Accept header matching
- Fix JSONCached to use proper integer formatting

internal/handlers/cv_contact.go

@@ -8,7 +8,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/juanatsap/cv-site/internal/constants"
+	c "github.com/juanatsap/cv-site/internal/constants"
 	"github.com/juanatsap/cv-site/internal/httputil"
 	"github.com/juanatsap/cv-site/internal/services"
 )
@@ -173,7 +173,7 @@ func (h *CVHandler) renderContactSuccess(w http.ResponseWriter, r *http.Request,
 	}
 
 	// Render the success template
-	w.Header().Set(constants.HeaderContentType, constants.ContentTypeHTML)
+	w.Header().Set(c.HeaderContentType, c.ContentTypeHTML)
 	w.WriteHeader(http.StatusOK)
 
 	tmpl, err := h.templates.Render("contact-success")
@@ -224,7 +224,7 @@ func (h *CVHandler) renderContactError(w http.ResponseWriter, r *http.Request, e
 	// Render the error template
 	// Return 200 OK with error content - HTMX 1.9.x logs console.error for non-2xx responses
 	// Validation errors are expected form feedback, not system errors
-	w.Header().Set(constants.HeaderContentType, constants.ContentTypeHTML)
+	w.Header().Set(c.HeaderContentType, c.ContentTypeHTML)
 	w.WriteHeader(http.StatusOK)
 
 	tmpl, err := h.templates.Render("contact-error")