feat: Cloudflare Turnstile on auth, CSP fixes, admin/SEO/analytics additions

Turnstile bot protection (sign-in, sign-up, password-reset):
- Register Better Auth's captcha plugin with the cloudflare-turnstile
  provider; endpoints listed explicitly rather than relying on defaults.
  /reset-password is intentionally excluded — it is reached only via a
  single-use emailed token.
- Add an explicit-render Turnstile widget component. Tokens are single-use,
  so each form resets the challenge after a failed submit; submit stays
  disabled until a token is held.
- Read the site key server-side and pass it down as a prop, so rotating it
  does not require a rebuild.
- Fail fast in production when TURNSTILE_SECRET_KEY is missing, and when a
  secret is set without a site key (that combination would demand a token
  no form can produce, locking every user out).
- Pass a throwaway secret during `next build` in the Dockerfile, mirroring
  the existing BETTER_AUTH_SECRET treatment, so image builds don't need it.

CSP fixes in middleware (these blocked Turnstile entirely):
- Add frame-src for challenges.cloudflare.com. Without it the widget's
  iframe fell back to default-src 'self' and was blocked outright.
- Allow 'unsafe-eval' and websockets in DEVELOPMENT only. `next dev`
  compiles with eval(), so the strict policy threw EvalError and killed
  hydration — no client JS ran at all, which also meant form submit
  handlers never fired. Production policy is unchanged and still strict.

Also included (concurrent work in the tree):
- Admin organizations pages and lib/admin/orgs.
- Episode moderation migration, SEO metadata (sitemap, robots, JSON-LD,
  OG/Twitter images, manifest), Umami analytics, not-found page.

Local dev database: docker-compose.dev.yml provisions Postgres 18 on port
5443 (5432-5442 are in use by other local projects).

Note: `npx tsc --noEmit` currently fails in app/(app)/team/page.tsx — an
`invitations` prop the component does not accept. This predates the commit
and will fail `next build` until fixed.

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

app/api/webhooks/paypal/route.ts

@@ -2,9 +2,20 @@ import { NextRequest } from "next/server";
 import { verifyPaypalWebhook } from "@/lib/billing/paypal";
 import { handlePaypalEvent } from "@/lib/billing/webhooks/paypal";
 import { alreadyProcessed, logWebhook } from "@/lib/billing/webhook-log";
+import { rateLimit, LIMITS } from "@/lib/ratelimit";
 
 export const dynamic = "force-dynamic";
 
+/** Best-effort client IP for anonymous rate limiting. */
+function clientKey(req: NextRequest): string {
+  const fwd = req.headers.get("x-forwarded-for");
+  if (fwd) return fwd.split(",")[0].trim();
+  return req.headers.get("x-real-ip") ?? "anon";
+}
+
+// PayPal webhook bodies are small; anything larger is not a real event.
+const MAX_BODY_BYTES = 1_000_000;
+
 const SIG_HEADERS = [
   "paypal-auth-algo",
   "paypal-cert-url",
@@ -14,9 +25,44 @@ const SIG_HEADERS = [
 ];
 
 export async function POST(req: NextRequest) {
-  const body = await req.text();
+  // This endpoint is unauthenticated and verification is done by CALLING PayPal,
+  // so every request costs us an outbound API call. Throttle per IP, and reject
+  // obviously-bogus requests before spending anything.
+  const rl = await rateLimit("paypal-webhook", clientKey(req), LIMITS.webhook);
+  if (!rl.ok) {
+    return new Response("Too many requests", {
+      status: 429,
+      headers: { "Retry-After": String(rl.retryAfterSec ?? 60) },
+    });
+  }
+
   const headers: Record<string, string | undefined> = {};
   for (const h of SIG_HEADERS) headers[h] = req.headers.get(h) ?? undefined;
+  // A genuine PayPal delivery always carries all five signature headers. Bailing
+  // here costs nothing, whereas verifyPaypalWebhook() would hit the network.
+  if (SIG_HEADERS.some((h) => !headers[h])) {
+    return new Response("Invalid signature", { status: 400 });
+  }
+
+  // Check the declared size BEFORE reading, so an oversized body is never
+  // materialized in memory. The post-read check still backstops a missing or
+  // lying Content-Length.
+  const declared = Number(req.headers.get("content-length") ?? "0");
+  if (Number.isFinite(declared) && declared > MAX_BODY_BYTES) {
+    return new Response("Payload too large", { status: 413 });
+  }
+
+  const body = await req.text();
+  if (body.length > MAX_BODY_BYTES) {
+    return new Response("Payload too large", { status: 413 });
+  }
+
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(body);
+  } catch {
+    return new Response("Invalid payload", { status: 400 });
+  }
 
   const verified = await verifyPaypalWebhook(headers, body).catch(() => false);
   if (!verified) {
@@ -24,7 +70,7 @@ export async function POST(req: NextRequest) {
     return new Response("Invalid signature", { status: 400 });
   }
 
-  const event = JSON.parse(body) as { id?: string; event_type?: string };
+  const event = parsed as { id?: string; event_type?: string };
   const eventId = event.id ?? `paypal_${Date.now()}`;
   if (event.id && (await alreadyProcessed(eventId))) return new Response("ok (duplicate)");