fix(rate-limit): key the guest-facing limiters per user, not per IP

At a venue every guest is behind one NAT, so an IP-keyed limiter hands the
whole party a single bucket. On a fresh deploy 12 guests arriving together
meant 5 joined and 7 were turned away, with no Retry-After telling them when
to retry. `/feed` (60/min) and `/export` (3 per DAY — the fourth guest to
fetch their keepsake locked out until tomorrow) had the same defect.

`feed_delta` was already keyed per-user and its comment states the exact
rationale ("so one client can't starve others behind a shared NAT"); this
makes its siblings match.

- feed:{ip}   -> feed:{user_id}    (auth was already in scope)
- export:{ip} -> export:{user_id}  (resolved from the download ticket's
  session, which was previously looked up and discarded)
- join:{ip}: pre-auth, so there is no user to key on. Split in two — a loose
  per-IP ceiling that only bounds raw volume (new `join_ip_rate_per_min`,
  default 60, migration 017), plus the real 5/60s anti-spam bucket keyed
  per (ip, name), mirroring the existing `recover:{ip}:{name}`.

admin_login / recover / pin_reset_req stay IP-keyed on purpose and are now
commented as such: they guard credential guessing, where a per-user or
per-name key would just hand an attacker a fresh bucket per guess.

Retry-After: the machinery existed but 7 of 8 sites called `check()` and
hard-coded `None`, so a throttled client was told to back off but never for
how long. Delete the bool `check()` wrapper entirely so `check_with_retry`
is the only entry point and the delay cannot be discarded by accident. Also
surface it for the PIN lockout, where the deadline was already known.

Fix the "unknown" fallback while here: every client_ip() caller passed that
literal, so any request without X-Forwarded-For — anything reaching the app
directly rather than through Caddy — shared ONE global bucket. Serve with
connect-info and use the peer address.

Tests: the reseed forces every limiter toggle off before each test, which is
why this whole class was invisible. Add 01-auth/rate-limit-shared-nat, which
enables them and asserts 12 guests share an IP without collision, that one
guest hammering their own name IS still throttled (so the fix re-keys rather
than removes the limit), and that feed/export buckets are per-user. Retarget
the ddos join test at the new per-IP ceiling — it asserted the defect.

Also seed `admin_login_rate_enabled` (read by the handler, seeded by no
migration and no reseed) and register `join_ip_rate_per_min` in the admin
config allowlist. Unrelated pre-existing red test fixed: 01-auth/join
asserted a "Willkommen!" heading the wedding redesign removed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/src/handlers/admin.rs

@@ -3,13 +3,13 @@ use std::time::Duration;
 
 use axum::Json;
 use axum::extract::{Query, State};
-use axum::http::{HeaderMap, StatusCode};
+use axum::http::StatusCode;
 use serde::{Deserialize, Serialize};
+use uuid::Uuid;
 
 use crate::auth::middleware::RequireAdmin;
 use crate::error::AppError;
 use crate::services::config;
-use crate::services::rate_limiter::client_ip;
 use crate::state::AppState;
 
 // ── DTOs ─────────────────────────────────────────────────────────────────────
@@ -120,6 +120,10 @@ pub async fn patch_config(
         ("upload_rate_per_hour", true, 1.0, 100_000.0),
         ("feed_rate_per_min", true, 1.0, 100_000.0),
         ("export_rate_per_day", true, 1.0, 100_000.0),
+        // Loose per-IP ceiling on /join. The real anti-spam bucket is per (ip, name); this
+        // only bounds raw volume from one source, so it must stay well above the size of a
+        // party arriving at once (see migration 017).
+        ("join_ip_rate_per_min", true, 1.0, 100_000.0),
         ("quota_tolerance", false, 0.0, 1.0),
         ("estimated_guest_count", true, 1.0, 1_000_000.0),
     ];
@@ -320,25 +324,26 @@ pub async fn export_ticket(
 }
 
 /// Validate a download ticket (single-use) and confirm its session still exists.
-async fn authenticate_download_ticket(state: &AppState, ticket: &str) -> Result<(), AppError> {
+/// Resolve a single-use download ticket to the user who minted it. The caller needs the
+/// id to key the export rate limit per-user (see `enforce_export_rate`).
+async fn authenticate_download_ticket(state: &AppState, ticket: &str) -> Result<Uuid, AppError> {
     let token_hash = state
         .sse_tickets
         .consume(ticket)
         .ok_or_else(|| AppError::Unauthorized("Ticket ungültig oder abgelaufen.".into()))?;
-    crate::models::session::Session::find_by_token_hash(&state.pool, &token_hash)
+    let session = crate::models::session::Session::find_by_token_hash(&state.pool, &token_hash)
         .await
         .map_err(|e| AppError::Internal(e.into()))?
         .ok_or_else(|| AppError::Unauthorized("Sitzung nicht gefunden.".into()))?;
-    Ok(())
+    Ok(session.user_id)
 }
 
 pub async fn download_zip(
     State(state): State<AppState>,
     Query(q): Query<DownloadQuery>,
-    headers: HeaderMap,
 ) -> Result<axum::response::Response, AppError> {
-    authenticate_download_ticket(&state, &q.ticket).await?;
-    enforce_export_rate(&state, &headers).await?;
+    let user_id = authenticate_download_ticket(&state, &q.ticket).await?;
+    enforce_export_rate(&state, user_id).await?;
 
     let path =
         resolve_export_file(&state, "zip", "Der ZIP-Export ist noch nicht verfügbar.").await?;
@@ -389,10 +394,9 @@ async fn resolve_export_file(
 pub async fn download_html(
     State(state): State<AppState>,
     Query(q): Query<DownloadQuery>,
-    headers: HeaderMap,
 ) -> Result<axum::response::Response, AppError> {
-    authenticate_download_ticket(&state, &q.ticket).await?;
-    enforce_export_rate(&state, &headers).await?;
+    let user_id = authenticate_download_ticket(&state, &q.ticket).await?;
+    enforce_export_rate(&state, user_id).await?;
 
     let path =
         resolve_export_file(&state, "html", "Der HTML-Export ist noch nicht verfügbar.").await?;
@@ -476,21 +480,24 @@ pub async fn export_status(
 /// Centralised guard for the export rate limit. Same pattern as upload/feed: master
 /// switch + per-endpoint switch + numeric value, all stored in `config` and read on
 /// each request.
-async fn enforce_export_rate(state: &AppState, headers: &HeaderMap) -> Result<(), AppError> {
+async fn enforce_export_rate(state: &AppState, user_id: Uuid) -> Result<(), AppError> {
     let rate_limits_on = config::get_bool(&state.config_cache, "rate_limits_enabled", true).await;
     let export_rate_on = config::get_bool(&state.config_cache, "export_rate_enabled", true).await;
     if !(rate_limits_on && export_rate_on) {
         return Ok(());
     }
-    let ip = client_ip(headers, "unknown");
     let limit = config::get_usize(&state.config_cache, "export_rate_per_day", 3).await;
-    if !state
-        .rate_limiter
-        .check(format!("export:{ip}"), limit, Duration::from_secs(86400))
-    {
+    // Keyed per-user. This was the worst of the IP-keyed limiters: 3 downloads per DAY
+    // shared across every guest behind the venue's public IP, so the fourth person to
+    // fetch their keepsake was locked out until the next day.
+    if let Err(retry_after_secs) = state.rate_limiter.check_with_retry(
+        format!("export:{user_id}"),
+        limit,
+        Duration::from_secs(86400),
+    ) {
         return Err(AppError::TooManyRequests(
             "Zu viele Anfragen. Bitte warte kurz und versuche es erneut.".into(),
-            None,
+            Some(retry_after_secs),
         ));
     }
     Ok(())