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

lib/billing/webhook-log.ts

@@ -3,7 +3,9 @@ import { prisma } from "@/lib/db";
 /** True if we've already handled this provider event (idempotency). */
 export async function alreadyProcessed(eventId: string): Promise<boolean> {
   const existing = await prisma.webhookEvent.findUnique({ where: { eventId } });
-  return !!existing;
+  // Only a successfully "processed" event is considered handled. Rows logged as
+  // "failed" (or "skipped") must be reprocessable when the provider retries.
+  return existing?.status === "processed";
 }
 
 /** Record a webhook delivery for the admin log (best-effort; unique on eventId). */