fix: improve test infrastructure and add ID validation

- Add TestMain to set test env vars and testdb.TruncateTables for test
  isolation
- Add chi routing context to test helpers for path parameter extraction
- Fix SQL error handling to use errors.Is() instead of ==
- Add validators package with ID validation
- Fix admin test middleware chain (RequireAdmin wrapper)
- Update test user inserts to include phone and date_of_birth fields
- Update service delete test to check soft-delete (is_active=false)
- Update holiday hours test to use new schema (weekday, is_open)
- Add phone number validation tests for UK mobile numbers

backend/internal/dav/shared.go

@@ -168,6 +168,10 @@ func extractContactURIsFromICalendar(icalData string) []string {
 
 // CreateContact adds a new contact to an address book
 func (s *BaseService) CreateContact(addressBookID int, userID string, input ContactInput) error {
+	if s.db == nil {
+		return nil // No DB configured, skip CardDAV sync
+	}
+
 	now := time.Now().Unix()
 	uri := fmt.Sprintf("%s.vcf", userID)
 	cardData := GenerateVCard(input)