security: stop trusting the leftmost x-forwarded-for for rate limits and abuse quotas (FRE-371)

getClientIP (the rate-limit and audit key) took the LEFTMOST x-forwarded-for
hop and read x-real-ip unconditionally. In our topology — client → Cloudflare
→ same-host origin proxy → app — both proxies APPEND, so the leftmost hop is
whatever the sender typed: a rotating X-Forwarded-For minted a fresh rate-limit
bucket per request, and unlimited bug-report quota via the same pattern in
routes/api/the-nook/report.ts. x-real-ip is client-settable too; nothing in
the chain overwrites it. It is no longer read at all.

New trust order (src/server/trusted-client-ip.ts, resolveTrustedClientIp):

  1. a non-loopback socket peer IS the client — its headers are ignored;
  2. cf-connecting-ip (Cloudflare overwrites it at the edge; parsed as an
     address or rejected);
  3. x-forwarded-for counted from the RIGHT, discounting one hop for the
     Cloudflare edge address the origin proxy appended (the topology has
     exactly one trusted proxy below Cloudflare);
  4. "unknown" — callers key stricter shared limits on it, so resolution
     fails closed toward a tighter bucket, never a looser one.

Addresses are never truncated: a /24-truncated key would pool attackers with
the innocents behind them.

Share-vs-copy, stated deliberately: this is a port of the parsing primitives
 FRE-366 reviewed in request-client.ts, NOT an import. resolveClientIp carries
a standing rule (1d9287a) that it never feeds a security decision; lifting
that rule for these callers would couple an access-control change to an
analytics branch under review, and the analytics helper only exposes
truncated prefixes while rate limiting needs the full address. If the two
drift, trusted-client-ip.ts is the one security paths follow. The deploy-side
precondition (origin reachable only via Cloudflare + the same-host proxy) is
documented in the file header; if the app is ever directly internet-reachable
no in-app read can recover the true client.

Tests updated deliberately, not to go green: rate-limit.test.ts now pins the
rightmost-minus-one order, cf-connecting-ip precedence, peer-over-headers,
x-real-ip rejection, and the spoof case (a rotating leftmost hop does not
change the resolved client); report.test.ts pins the quota key stability
under a rotating header. New trusted-client-ip.test.ts covers the helper's
own edges: APIEvent wrapper, loopback forms, unparseable hops failing closed.

src/server/security.ts

@@ -3,6 +3,7 @@ import { getCookie, setCookie } from "vinxi/http";
 import type { H3Event } from "vinxi/http";
 import { t } from "~/server/api/utils";
 import { logAuditEvent } from "~/server/audit";
+import { resolveTrustedClientIp } from "~/server/trusted-client-ip";
 import { env } from "~/env/server";
 import {
   AUTH_CONFIG,
@@ -328,39 +329,15 @@ async function cleanupExpiredRateLimits(): Promise<void> {
 }
 
 /**
- * Get client IP address from request headers.
- * Only trusts X-Forwarded-For outside of local development (set by the Vercel
- * edge network in production; trusted in tests so the header-parsing path is
- * exercised). In local development, uses the socket address to prevent header
- * spoofing.
+ * Get client IP address for a security decision (rate limiting, audit).
+ *
+ * Thin alias over `resolveTrustedClientIp` (./trusted-client-ip), which owns
+ * the trust order: the previous leftmost `x-forwarded-for` read was
+ * client-spoofable in our append-only proxy topology, and `x-real-ip` was
+ * read unconditionally — both fixed there (FRE-371).
  */
 export function getClientIP(event: H3Event): string {
-  // In production on Vercel, X-Forwarded-For is set by the edge network
-  // and cannot be spoofed by clients. In dev, ignore it.
-  if (env.NODE_ENV !== "development") {
-    const forwarded = getHeaderValue(event, "x-forwarded-for");
-    if (forwarded) {
-      return forwarded.split(",")[0].trim();
-    }
-    const realIP = getHeaderValue(event, "x-real-ip");
-    if (realIP) {
-      return realIP;
-    }
-  }
-
-  // Fallback: try socket remote address
-  try {
-    const nodeReq = event.node.req;
-    if (nodeReq?.socket?.remoteAddress) {
-      const addr = nodeReq.socket.remoteAddress;
-      // Clean up IPv6-mapped IPv4 addresses
-      return addr.replace(/^::ffff:/, "");
-    }
-  } catch {
-    // socket access failed
-  }
-
-  return "unknown";
+  return resolveTrustedClientIp(event);
 }
 
 /**