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/httputil/request.go

@@ -4,15 +4,15 @@ package httputil
 import (
 	"net/http"
 
-	"github.com/juanatsap/cv-site/internal/constants"
+	c "github.com/juanatsap/cv-site/internal/constants"
 )
 
 // Lang extracts and validates the language from query params.
 // Returns the language or default if invalid/missing.
 func Lang(r *http.Request) string {
 	lang := r.URL.Query().Get("lang")
-	if lang == "" || !constants.SupportedLanguages[lang] {
-		return constants.LangDefault
+	if lang == "" || !c.SupportedLanguages[lang] {
+		return c.LangDefault
 	}
 	return lang
 }
@@ -22,9 +22,9 @@ func Lang(r *http.Request) string {
 func LangOrError(r *http.Request) (string, bool) {
 	lang := r.URL.Query().Get("lang")
 	if lang == "" {
-		return constants.LangDefault, true
+		return c.LangDefault, true
 	}
-	if !constants.SupportedLanguages[lang] {
+	if !c.SupportedLanguages[lang] {
 		return "", false
 	}
 	return lang, true