test: add comprehensive Go test suite with ~75% coverage

New test files:
- config/config_test.go (100% coverage)
- constants/constants_test.go (100% coverage)
- httputil/response_test.go (100% coverage)
- validation/rules_test.go (91.9% coverage)
- middleware/logger_test.go, security_test.go, security_logger_test.go
- handlers/errors_test.go

Updated documentation:
- doc/27-GO-TESTING.md: Complete testing guide
- doc/00-GO-DOCUMENTATION-INDEX.md: Added testing section
- doc/01-ARCHITECTURE.md: Updated package structure
- doc/DECISIONS.md: Added ADR-004 caching decision
- PROJECT-MEMORY.md: Added Go testing section

doc/01-ARCHITECTURE.md

@@ -17,12 +17,17 @@ This CV website is built following Go best practices with a focus on:
 cv/
 ├── main.go                    # Application entry point
 └── internal/                  # Private packages (cannot be imported by other projects)
+    ├── cache/                # Application-level data caching
     ├── config/               # Configuration management
+    ├── constants/            # Project-wide constants
+    ├── email/                # Email service (SMTP)
+    ├── fileutil/             # File path utilities
     ├── handlers/             # HTTP request handlers
+    ├── httputil/             # HTTP response helpers
     ├── middleware/           # HTTP middleware (security, logging, rate limiting)
-    ├── models/               # Data models and business logic
+    ├── models/               # Data models (cv, ui)
     ├── pdf/                  # PDF generation service
-    ├── services/             # Business services (email, etc.)
+    ├── routes/               # Route configuration
     ├── templates/            # Template management
     └── validation/           # Input validation utilities
 ```
@@ -41,13 +46,15 @@ Handlers and services receive their dependencies through constructors:
 // ✅ Good: Dependencies injected
 type CVHandler struct {
     templates    *templates.Manager
-    emailService *services.EmailService
+    emailService *email.Service
+    dataCache    *cache.DataCache
 }
 
-func NewCVHandler(tmpl *templates.Manager, addr string, email *services.EmailService) *CVHandler {
+func NewCVHandler(tmpl *templates.Manager, addr string, emailSvc *email.Service, dc *cache.DataCache) *CVHandler {
     return &CVHandler{
         templates:    tmpl,
-        emailService: email,  // Can be nil for graceful degradation
+        emailService: emailSvc,   // Can be nil for graceful degradation
+        dataCache:    dc,         // Startup-loaded data cache
     }
 }
 
@@ -170,7 +177,7 @@ func (h *CVHandler) HandleContact(w http.ResponseWriter, r *http.Request)
 - Consistent error handling
 - HTMX-aware responses
 
-### Email Service (`internal/services`)
+### Email Service (`internal/email`)
 
 **Pattern**: Service layer with dependency injection and interface-based design