fix(rsvp): defend the edit flow against forwarded invitation links

When a guest submitted via their invitation and then forwarded the link
(or someone copied the URL), the recipient was shown the original guest's
response and a "Change my response" button. Two real problems:
  - Privacy leak: the original guest's reply was visible
  - Integrity: the recipient could silently overwrite the response

The fix is two layered defences plus a recovery path, matching the
industry pattern used by Eventbrite / Partiful / Lu.ma:

Backend
  - GET /access/{token} now compares the device fingerprint of the
    current request to the fingerprint stored on the existing RSVP.
    When they don't match, the rsvp field is omitted from the
    response and a new rsvp_submitted_elsewhere flag is set instead.
    The original guest's reply stays private.
  - PATCH /rsvp/{token} runs the same gate before scoring. A foreign
    device gets 403 with a hint to request an edit link.
  - The fingerprint check is intentionally narrow (user_agent only),
    so a guest jumping between Wi-Fi and mobile data on the same
    phone still sails through.

Recovery path
  - New POST /access/{token}/request-edit-link mints a short-lived
    edit nonce (Redis, 30-min TTL, SHA-256-hashed), then emails it
    to the guest's address on file via the existing EmailSender.
    Rate-limited to 3 per token per hour.
  - GET /access/{token}?edit=<nonce> and PATCH /rsvp with edit_nonce
    in the body both accept the nonce as a bypass for the
    same-device check. Lets the real guest edit from a new phone
    when their original device is gone.
  - New SendRSVPEditLink method on auth.EmailSender, implemented by
    every concrete sender (log stub / Resend / SMTP / SES), with a
    branded HTML+text template that explains the "we sent this
    because we didn't recognise the device" framing.

Frontend
  - rsvp/[token].vue learns the new "responded elsewhere" state.
    Renders "This invitation has already been used" + a
    "Send me an edit link" CTA when the access response says we
    have somewhere to deliver it. Empty-state copy reads "If you
    forwarded the link, please ask the original guest to reach
    out to the host".
  - When the URL carries ?edit=<nonce>, the page passes it on the
    /access call (so the backend unhides the RSVP), opens the edit
    form pre-populated, and forwards the nonce on PATCH.
  - Removed two leftover leaks from earlier — the page no longer
    shows internal "Risk score N · band" to confirmed or blocked
    guests; the blocked-attempt copy now reads "Something about
    this attempt looked off" rather than "suspicious access
    attempt".

Defensive nil-guard
  - The access handler's NATS publish goroutine now skips when
    deps.AccessPublisher is nil (matches the rsvp publisher's
    existing guard); without it the handler nil-panicked in tests
    that don't wire NATS.

Tests
  - TestFingerprintsSimilar (unit) covers the same-UA / different-UA
    / missing-UA matrix.
  - TestForwardedInvitationLinkDefence (integration) walks the full
    flow: submit from UA-A, hide on UA-B, request link, follow nonce
    from UA-B and edit, then verify a UA-C with a forged nonce is
    still refused.
  - Full integration suite passes (183.5s).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/api/rsvp_edit_test.go

@@ -0,0 +1,52 @@
+package api
+
+import "testing"
+
+func TestFingerprintsSimilar(t *testing.T) {
+	const ua = "Mozilla/5.0 (Macintosh; Intel Mac OS X 14_2) AppleWebKit/605"
+	other := "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537"
+
+	tests := []struct {
+		name           string
+		stored, current map[string]any
+		want           bool
+	}{
+		{
+			name:    "identical UA, different IPs counts as same device",
+			stored:  map[string]any{"user_agent": ua, "accept_language": "en"},
+			current: map[string]any{"user_agent": ua, "accept_language": "fr"},
+			want:    true,
+		},
+		{
+			name:    "different UA never matches",
+			stored:  map[string]any{"user_agent": ua},
+			current: map[string]any{"user_agent": other},
+			want:    false,
+		},
+		{
+			name:    "missing UA on current side is treated as mismatch",
+			stored:  map[string]any{"user_agent": ua},
+			current: map[string]any{},
+			want:    false,
+		},
+		{
+			name:    "missing UA on stored side is treated as mismatch",
+			stored:  map[string]any{},
+			current: map[string]any{"user_agent": ua},
+			want:    false,
+		},
+		{
+			name:    "both nil is mismatch (conservative default)",
+			stored:  nil,
+			current: nil,
+			want:    false,
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := fingerprintsSimilar(tc.stored, tc.current); got != tc.want {
+				t.Errorf("fingerprintsSimilar = %v, want %v", got, tc.want)
+			}
+		})
+	}
+}