Notify question authors via Resend when an admin posts the first answer.

Require email at registration (editable on profile), add a Resend mailer with idempotent first-answer sends, and keep answer saves independent of delivery.

internal/store/email_test.go

@@ -0,0 +1,21 @@
+package store
+
+import "testing"
+
+func TestValidateEmail(t *testing.T) {
+	cases := []struct {
+		in, want, err string
+	}{
+		{"", "", "Email is required."},
+		{"  Alice@Example.COM ", "alice@example.com", ""},
+		{"not-an-email", "", "Enter a valid email address."},
+		{"a@b", "", "Enter a valid email address."},
+		{"ok@example.com", "ok@example.com", ""},
+	}
+	for _, tc := range cases {
+		got, msg := ValidateEmail(tc.in)
+		if got != tc.want || msg != tc.err {
+			t.Fatalf("%q: got (%q, %q) want (%q, %q)", tc.in, got, msg, tc.want, tc.err)
+		}
+	}
+}