Add E.164 input validation for phone numbers (FRE-4506)

- Extract phone validation to reusable utility (src/utils/phone-validation.ts)
- Use libphonenumber-js for strict E.164 format validation
- Normalize accepted numbers to canonical E.164 format
- Add 22 comprehensive validation tests covering valid/invalid formats
- Update existing tests to use valid E.164 test numbers
- All 56 tests passing

Co-Authored-By: Paperclip <noreply@paperclip.ing>

services/spamshield/src/utils/phone-validation.ts

@@ -0,0 +1,25 @@
+import { isValidPhoneNumber, parsePhoneNumber } from 'libphonenumber-js';
+
+export class PhoneNumberValidationError extends Error {
+  constructor(public readonly originalInput: string) {
+    super(
+      `Invalid E.164 phone number format: ${originalInput}. Expected format: +[country code][number] (e.g., +14155552671)`
+    );
+    this.name = 'PhoneNumberValidationError';
+  }
+}
+
+export function validatePhoneNumber(phoneNumber: string): string {
+  const trimmed = phoneNumber.trim();
+
+  if (!isValidPhoneNumber(trimmed)) {
+    throw new PhoneNumberValidationError(phoneNumber);
+  }
+
+  const parsed = parsePhoneNumber(trimmed);
+  if (!parsed || !parsed.number) {
+    throw new PhoneNumberValidationError(phoneNumber);
+  }
+
+  return parsed.number;
+}