refactor: centralize constants and reorganize documentation

- Create internal/constants package with all hardcoded values
  (environment, cookies, themes, headers, routes, cache)
- Create internal/httputil package for HTTP helper functions
- Update all handlers and middleware to use centralized constants
- Reorganize documentation with numbered prefixes (00-26)
- Remove duplicate docs from validation folder and docs/
- Delete handlers/constants.go (moved to internal/constants)

internal/handlers/health.go

@@ -5,6 +5,8 @@ import (
 	"log"
 	"net/http"
 	"time"
+
+	"github.com/juanatsap/cv-site/internal/constants"
 )
 
 // HealthResponse represents the health check response
@@ -34,7 +36,7 @@ func (h *HealthHandler) Check(w http.ResponseWriter, r *http.Request) {
 		Version:   h.version,
 	}
 
-	w.Header().Set("Content-Type", "application/json")
+	w.Header().Set(constants.HeaderContentType, constants.ContentTypeJSON)
 	w.WriteHeader(http.StatusOK)
 	if err := json.NewEncoder(w).Encode(response); err != nil {
 		log.Printf("ERROR encoding health check response: %v", err)