security: fix full audit findings (v0.32.0)

Full list of the audit's confirmed findings and their fixes:

- Stored XSS via unescaped JSON-LD on the public recipe page
  (app/r/[id]/page.tsx) — escape < before injecting.
- CSP allowed unsafe-eval in production — now dev-only (Next prod
  never eval()s; only its HMR does).
- avatarUrl accepted any URL with no ownership check — now takes an
  avatarKey issued by avatar-presign, validated server-side, same
  pattern as recipe/review photos.
- No session revocation on password change/reset — both now revoke
  other sessions (revokeOtherSessions: true, revokeSessionsOnPasswordReset).
- Rate-limit bypass via spoofable X-Forwarded-For — take the last
  (proxy-appended) hop instead of the first (client-supplied) one,
  matching the single-Traefik-hop topology.
- Webhook signing secrets stored plaintext — now AES-256-GCM
  encrypted like every other secret in this app, with a legacy-
  plaintext fallback for pre-existing rows (bare hex has no ":", our
  ciphertext format always does).
- Better Auth's own rate limiter defaulted to in-memory storage,
  ineffective across replicas — now backed by the same Redis as
  lib/rate-limit.ts (secondaryStorage), with storeSessionInDatabase
  explicit so session storage itself doesn't move as a side effect.
- Presigned upload URLs didn't bind the declared file size to the
  actual upload, letting a client under-declare size (and quota
  charge) then PUT an arbitrarily large object — switched to S3
  presigned POST with a signed content-length-range condition,
  enforced by the storage server itself.
- generateMetadata() on the recipe page skipped the visibility
  filter the page body uses, leaking a private recipe's title via
  <title> to any signed-in user with the id.
- Block/unblock had no rate limit, unlike follow/unfollow.
- AI quota was charged even when a user's own BYOK key was used
  (their own credentials/billing) — added an isByok flag through
  the config-resolution chain and skip the charge when set. Also
  wired BYOK into generate/generate-from-idea/translate/import-url,
  which never looked it up at all before.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/lib/auth/server.ts

@@ -6,10 +6,32 @@ import { sendEmail, verifyEmailHtml, resetPasswordHtml, welcomeHtml } from "@/li
 import { isSignupsDisabled } from "@/lib/site-settings";
 import { findValidInvite, consumeInvite, INVITE_COOKIE } from "@/lib/invites";
 import { generateUniqueUsername } from "@/lib/username";
+import { getRedis } from "@/lib/redis";
+
+// Without this, better-auth's own rate limiter (sign-in/sign-up/2FA/password
+// reset throttles below) defaults to an in-process Map — fine for a single
+// instance, but each replica gets its own independent counter the moment
+// this app is ever scaled horizontally, multiplying the effective limit by
+// the instance count. Backing it with the same Redis used elsewhere
+// (lib/rate-limit.ts) makes the limit actually shared and enforceable.
+const redisSecondaryStorage = {
+  async get(key: string) {
+    return getRedis().get(key);
+  },
+  async set(key: string, value: string, ttl?: number) {
+    if (ttl) await getRedis().set(key, value, "EX", ttl);
+    else await getRedis().set(key, value);
+  },
+  async delete(key: string) {
+    await getRedis().del(key);
+  },
+};
 
 export const auth = betterAuth({
   trustedOrigins: [process.env["BETTER_AUTH_URL"] ?? "http://localhost:3000"],
 
+  secondaryStorage: redisSecondaryStorage,
+
   // Explicit rather than relying on the isProduction default so dev/staging
   // are protected too. Sign-in/sign-up/change-password/change-email get a
   // strict built-in 3-req/10s-per-IP rule (better-auth's default special
@@ -34,6 +56,9 @@ export const auth = betterAuth({
   emailAndPassword: {
     enabled: true,
     requireEmailVerification: true,
+    // A stolen session should not survive the account owner noticing and
+    // resetting their password.
+    revokeSessionsOnPasswordReset: true,
     sendResetPassword: async ({ user, url }) => {
       await sendEmail({
         to: user.email,
@@ -100,6 +125,12 @@ export const auth = betterAuth({
       enabled: true,
       maxAge: 60 * 5,
     },
+    // secondaryStorage above is only meant to back the rate limiter — without
+    // this, better-auth would also move session storage itself into Redis
+    // (see internal-adapter.mjs's `storeInDb` checks), which is an unrelated,
+    // much bigger behavior change (session data no longer queryable from the
+    // `sessions` table at all) that nothing else in this app expects.
+    storeSessionInDatabase: true,
   },
 
   databaseHooks: {