test(backend): add phone injection-security tests

Verify ValidateUKPhoneNumber rejects SQLi, XSS, command injection, and control character payloads. Also verifies mixed injection-wrapped numbers are rejected (libphonenumber doesn't extract digits from noise).

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/auth/auth_test.go

@@ -1469,5 +1469,59 @@ func TestLoginInProgress_Cap(t *testing.T) {
 	}
 }
 
+// ValidateUKPhoneNumber Security Tests
+//
+// These tests verify that ValidateUKPhoneNumber rejects or sanitises
+// injection payloads (SQLi, XSS, command injection, control characters).
+
+func TestValidateUKPhoneNumber_RejectsPureInjectionPayloads(t *testing.T) {
+	payloads := []string{
+		// SQL injection
+		"' OR '1'='1",
+		"admin'--",
+		`" OR 1=1 --`,
+		"'; DROP TABLE users;--",
+		// XSS
+		"<script>alert(1)</script>",
+		"<img src=x onerror=alert(1)>",
+		"\"><script>alert(1)</script>",
+		"javascript:alert(1)",
+		// Command injection
+		"; rm -rf /",
+		"| cat /etc/passwd",
+		"`id`",
+		"$(cat /etc/passwd)",
+		// Control characters
+		"\n",
+		"\r\n",
+		"\x00",
+	}
+	for _, p := range payloads {
+		result, err := ValidateUKPhoneNumber(p)
+		if err == nil {
+			t.Errorf("expected injection payload %q to be rejected, got result %q", p, result)
+		}
+	}
+}
+
+func TestValidateUKPhoneNumber_RejectsMixedInjectionPayloads(t *testing.T) {
+	// When injection characters are interleaved with a valid UK phone number,
+	// libphonenumber rejects the entire input — it does NOT try to extract
+	// digits from non-numeric characters. This is MORE secure than naive
+	// digit-stripping approaches.
+	payloads := []string{
+		"' OR '1'='1 OR '+447700900000",
+		"><script>+447700900000</script>",
+		"'; rm -rf /; +447700900000",
+		"\x00\n+447700900000",
+	}
+	for _, p := range payloads {
+		result, err := ValidateUKPhoneNumber(p)
+		if err == nil {
+			t.Errorf("expected mixed payload %q to be rejected, got result %q", p, result)
+		}
+	}
+}
+
 // Ensure test compilation - import pgxpool to avoid unused import
 var _ = func() *pgxpool.Pool { return nil }