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>

lib/billing/paypal.ts

@@ -24,7 +24,24 @@ export function isPaypalConfigured(): boolean {
   return !!(process.env.PAYPAL_CLIENT_ID && process.env.PAYPAL_CLIENT_SECRET);
 }
 
+// PayPal client-credentials tokens are valid for hours. Minting a fresh one per
+// call turned every inbound webhook into two outbound PayPal requests, which an
+// anonymous caller could amplify until our PayPal rate limits were exhausted.
+// Cache it in-process and refresh a minute before expiry.
+let tokenCache: { token: string; expiresAt: number } | null = null;
+let tokenInFlight: Promise<string> | null = null;
+
 async function accessToken(): Promise<string> {
+  if (tokenCache && Date.now() < tokenCache.expiresAt) return tokenCache.token;
+  // Collapse concurrent misses onto a single token request.
+  if (tokenInFlight) return tokenInFlight;
+  tokenInFlight = fetchAccessToken().finally(() => {
+    tokenInFlight = null;
+  });
+  return tokenInFlight;
+}
+
+async function fetchAccessToken(): Promise<string> {
   const { id, secret } = creds();
   const res = await fetch(`${base()}/v1/oauth2/token`, {
     method: "POST",
@@ -35,7 +52,10 @@ async function accessToken(): Promise<string> {
     body: "grant_type=client_credentials",
   });
   if (!res.ok) throw new Error(`PayPal token error ${res.status}`);
-  const data = (await res.json()) as { access_token: string };
+  const data = (await res.json()) as { access_token: string; expires_in?: number };
+  // Default to 5 minutes if PayPal omits expires_in; refresh 60s early.
+  const ttlSec = Math.max((data.expires_in ?? 300) - 60, 60);
+  tokenCache = { token: data.access_token, expiresAt: Date.now() + ttlSec * 1000 };
   return data.access_token;
 }