fix: per-IP auth rate limiting instead of one global bucket

A single global token bucket let one attacker at the sustained rate 429
every user's login/register/change-password. Key buckets by client IP: the
SvelteKit proxy now stamps the real peer address onto X-Forwarded-For
(overriding any client-supplied value, anti-spoof), and axum reads it via a
ClientIp extractor — but only when AUTH_TRUSTED_PROXY is set, else it falls
back to the shared bucket (today's behavior). The per-IP map is bounded
(10k IPs, idle buckets pruned) so a spoofed-IP spray can't grow it.

AUTH_TRUSTED_PROXY defaults false (safe for a directly-exposed backend);
compose sets it true since the proxy is the single trusted hop.

Bump to 0.124.12.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/config.rs

@@ -27,6 +27,14 @@ pub struct AuthConfig {
     /// so a private instance is locked down with a single switch.
     /// Defaults to `false` (current public behaviour).
     pub private_mode: bool,
+    /// Whether to trust a proxy-supplied `X-Forwarded-For` header as the
+    /// client IP for per-IP auth rate limiting. Enable ONLY when the backend
+    /// sits behind a trusted reverse proxy that overrides the header (the
+    /// compose deploy: SvelteKit's hooks.server.ts sets it from the real peer
+    /// address). When `false` (default), the header is ignored and the auth
+    /// limiter uses a single shared bucket — a directly-exposed backend must
+    /// keep this off or clients could spoof their IP to dodge the limit.
+    pub trusted_proxy: bool,
 }
 
 impl Default for AuthConfig {
@@ -42,6 +50,7 @@ impl Default for AuthConfig {
             rate_limit: crate::auth::rate_limit::RateLimitConfig::default(),
             allow_self_register: true,
             private_mode: false,
+            trusted_proxy: false,
         }
     }
 }
@@ -584,6 +593,7 @@ impl Config {
                 },
                 allow_self_register: env_bool("ALLOW_SELF_REGISTER", true),
                 private_mode: env_bool("PRIVATE_MODE", false),
+                trusted_proxy: env_bool("AUTH_TRUSTED_PROXY", false),
             },
             upload: UploadConfig {
                 max_request_bytes: env_usize("MAX_REQUEST_BYTES", 200 * 1024 * 1024),