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/validators/validators.go

@@ -0,0 +1,17 @@
+package validators
+
+import (
+	"regexp"
+)
+
+// ID format: 12-character hexadecimal string (from gen_random_bytes(6) encoded as hex)
+var validIDRegex = regexp.MustCompile(`^[0-9a-f]{12}$`)
+
+// IsValidID checks if an ID is valid based on the database constraint (CHAR(12) hex string)
+// Valid IDs are exactly 12 hexadecimal characters (0-9, a-f)
+func IsValidID(id string) bool {
+	if id == "" {
+		return false
+	}
+	return validIDRegex.MatchString(id)
+}