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>

.env.example

@@ -51,6 +51,13 @@ SESSION_TTL_DAYS=30
 # rate-limiting reverse proxy that already enforces a budget).
 AUTH_RATE_PER_SEC=5
 AUTH_RATE_BURST=10
+# Trust a proxy-supplied X-Forwarded-For as the client IP for per-IP auth
+# rate limiting. Enable ONLY when the backend sits behind a trusted proxy
+# that overrides the header (the compose deploy: SvelteKit forwards the real
+# peer IP). When false, the header is ignored and a single shared bucket is
+# used — a directly-exposed backend MUST keep this off or clients could spoof
+# their IP to dodge the limit.
+AUTH_TRUSTED_PROXY=false
 
 # ----- CORS -----
 # Comma-separated origins allowed to call the API with credentials.