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/webhooks/stripe.ts

@@ -28,9 +28,13 @@ async function syncStripeSubscription(
   const item = sub.items.data[0];
   const priceId = item?.price?.id;
   const mapped = priceId ? planFromStripePrice(priceId) : null;
-  // metadata.plan is attacker-influenceable; only honour it if it's a known plan.
-  // The price-mapping fallback (derived from the real Stripe price) is preferred.
-  const plan: PlanKey = planFromMetadata(metadata?.plan) ?? mapped?.plan ?? "free";
+  // The PRICE is authoritative: it is what the customer is actually charged, and
+  // Stripe updates it on every plan change. metadata.plan is only written once at
+  // checkout (lib/billing/stripe.ts) and is NOT rewritten when a customer switches
+  // plans in the Billing Portal — trusting it first would let a downgraded customer
+  // keep the higher tier's entitlements. Metadata is a fallback for the case where
+  // a price is missing or unmapped, and is still narrowed to a known PlanKey.
+  const plan: PlanKey = mapped?.plan ?? planFromMetadata(metadata?.plan) ?? "free";
   const referenceId = metadata?.subjectId || sub.metadata?.subjectId;
   if (!referenceId || referenceId.trim() === "") {
     console.warn("[stripe] subscription without subjectId metadata, skipping", sub.id);