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/robots.ts

@@ -0,0 +1,42 @@
+import type { MetadataRoute } from "next";
+import { SITE_URL, absoluteUrl } from "@/lib/seo";
+
+/**
+ * /robots.txt — generated so the host always matches the deployment origin.
+ *
+ * Everything behind auth (the app shell, admin, auth screens), the API surface,
+ * and unlisted share links are disallowed. Those routes also emit a `noindex`
+ * meta tag; robots.txt keeps crawlers from spending budget on them at all.
+ */
+export default function robots(): MetadataRoute.Robots {
+  return {
+    rules: [
+      {
+        userAgent: "*",
+        allow: "/",
+        disallow: [
+          "/api/",
+          "/admin",
+          "/dashboard",
+          "/episodes",
+          "/series",
+          "/usage",
+          "/billing",
+          "/team",
+          "/api-keys",
+          "/settings",
+          "/sign-in",
+          "/sign-up",
+          "/forgot-password",
+          "/reset-password",
+          // Unlisted, per-episode share links — shareable, never indexable.
+          "/p/",
+          // Proxied analytics tracker + beacon (see next.config.mjs).
+          "/_a/",
+        ],
+      },
+    ],
+    sitemap: absoluteUrl("/sitemap.xml"),
+    host: SITE_URL,
+  };
+}