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/opengraph-image.tsx

@@ -0,0 +1,89 @@
+import { readFile } from "node:fs/promises";
+import { join } from "node:path";
+import { ImageResponse } from "next/og";
+import { BRAND_HEX, SITE_NAME } from "@/lib/seo";
+
+/**
+ * The default social card for every route that does not define its own.
+ * Rendered at build/request time by Satori — no binary asset to keep in sync.
+ */
+export const alt = `${SITE_NAME} — from a topic idea to a finished podcast in minutes`;
+export const size = { width: 1200, height: 630 };
+export const contentType = "image/png";
+
+/**
+ * The real brand mark, inlined as a data URI. Satori cannot fetch a relative URL
+ * (there is no origin while rendering), so the PNG is read off disk and embedded.
+ */
+async function brandMarkDataUri() {
+  const file = await readFile(join(process.cwd(), "app", "icon.png"));
+  return `data:image/png;base64,${file.toString("base64")}`;
+}
+
+export default async function OpengraphImage() {
+  const mark = await brandMarkDataUri();
+
+  return new ImageResponse(
+    (
+      <div
+        style={{
+          width: "100%",
+          height: "100%",
+          display: "flex",
+          flexDirection: "column",
+          justifyContent: "space-between",
+          background: "#0d0d0d",
+          padding: 72,
+        }}
+      >
+        {/* Brand accent — a soft corner glow. Satori has no blur filter, so the
+            falloff is a radial gradient rather than an opaque disc, which would
+            otherwise cut a hard edge straight through the headline. */}
+        <div
+          style={{
+            position: "absolute",
+            top: -340,
+            right: -300,
+            width: 900,
+            height: 900,
+            borderRadius: 9999,
+            background: `radial-gradient(circle, ${BRAND_HEX}cc 0%, ${BRAND_HEX}33 45%, ${BRAND_HEX}00 70%)`,
+            display: "flex",
+          }}
+        />
+
+        <div style={{ display: "flex", alignItems: "center", gap: 20 }}>
+          {/* eslint-disable-next-line @next/next/no-img-element */}
+          <img src={mark} alt="" width={64} height={64} />
+          <div style={{ display: "flex", fontSize: 30, fontWeight: 700, color: "#ffffff" }}>
+            {SITE_NAME}
+          </div>
+        </div>
+
+        <div style={{ display: "flex", flexDirection: "column", gap: 24 }}>
+          <div
+            style={{
+              display: "flex",
+              fontSize: 76,
+              fontWeight: 800,
+              lineHeight: 1.08,
+              letterSpacing: -2,
+              color: "#ffffff",
+              maxWidth: 940,
+            }}
+          >
+            From a topic idea to a finished podcast in minutes
+          </div>
+          <div style={{ display: "flex", fontSize: 30, color: "#a3a3a3", maxWidth: 900 }}>
+            AI writes the script, records multi-voice audio, and designs the cover art.
+          </div>
+        </div>
+
+        <div style={{ display: "flex", fontSize: 26, color: BRAND_HEX, fontWeight: 600 }}>
+          Script · Voice · Cover art — one workflow
+        </div>
+      </div>
+    ),
+    size
+  );
+}