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/(admin)/layout.tsx

@@ -1,3 +1,4 @@
+import type { Metadata } from "next";
 import Link from "next/link";
 import { ArrowLeft } from "lucide-react";
 import { requireAdmin } from "@/lib/auth/guards";
@@ -6,10 +7,15 @@ import { AdminMobileNav } from "@/components/admin/admin-mobile-nav";
 import { UserMenu } from "@/components/app/user-menu";
 import { Button } from "@/components/ui/button";
 import { Logo } from "@/components/ui/logo";
+import { NO_INDEX } from "@/lib/seo";
 
 // Authed, DB-backed admin surface — never statically prerender.
 export const dynamic = "force-dynamic";
 
+// Authenticated surface — never indexable. Metadata merges down, so every route
+// in this group inherits `noindex, nofollow` unless it explicitly overrides it.
+export const metadata: Metadata = NO_INDEX;
+
 export default async function AdminLayout({ children }: { children: React.ReactNode }) {
   const session = await requireAdmin();