Security & robustness hardening pass

Cross-cutting input-validation, isolation, and DoS-resistance fixes across
the app, API, billing, queue, and infra layers.

- Runtime validation (zod) for client-supplied admin actions (role/plan/
  limits), series generation index, and all pg-boss queue payloads
- Auth: require email verification before sign-in; reject weak/placeholder/
  short BETTER_AUTH_SECRET in production
- Billing: sanitize Stripe/PayPal errors (log server-side, generic to client);
  race-safe subscription upsert; only count "processed" webhook events as
  handled; verify org membership in getEffectivePlan to block plan escalation
- Series generation: reserve usage up front and refund on failure; bill the
  owning org, not the caller's active org
- Injection defenses: HTML-escape user fields in emails, strip CR/LF from
  subject/recipient, validate ElevenLabs voiceId before URL interpolation
- Media routes: stream off disk instead of buffering whole files; rate-limit
  anonymous public audio/cover endpoints by client IP

app/api/assets/[...key]/route.ts

@@ -1,3 +1,4 @@
+import { Readable } from "node:stream";
 import { NextRequest } from "next/server";
 import { getServerSession } from "@/lib/auth/guards";
 import { prisma } from "@/lib/db";
@@ -42,20 +43,22 @@ export async function GET(
   const isAdmin = session.user.role === "admin";
   if (!isOwner && !isAdmin) return new Response("Forbidden", { status: 403 });
 
-  const exists = await storage().exists(key);
-  if (!exists) return new Response("Not found", { status: 404 });
+  // Stream off disk instead of buffering the whole file into memory.
+  const total = await storage().size(key);
+  if (total === null) return new Response("Not found", { status: 404 });
 
-  const data = await storage().get(key);
   const ext = key.split(".").pop()?.toLowerCase() ?? "";
   const contentType = CONTENT_TYPES[ext] ?? "application/octet-stream";
 
   const download = req.nextUrl.searchParams.get("download");
   const filename = key.split("/").pop() ?? "asset";
 
-  return new Response(data as BodyInit, {
+  const node = storage().createReadStream!(key);
+  const body = Readable.toWeb(node as Readable) as unknown as BodyInit;
+  return new Response(body, {
     headers: {
       "Content-Type": contentType,
-      "Content-Length": String(data.byteLength),
+      "Content-Length": String(total),
       "Cache-Control": "private, max-age=3600",
       ...(download
         ? { "Content-Disposition": `attachment; filename="${filename}"` }