feat(billing): let people try the product before handing over a card

The trial now starts inside the product, at email verification, and Stripe
grants none at all: checkout creates a subscription that bills immediately.
Verifying an address is what buys the seven days, which is also the cheapest
abuse control there is.

An unexpired trial is treated as an entitlement the account already holds, so a
Stripe sync can add access but never retracts a trial that has not run out. That
matters most for the abandoned checkout: the resulting incomplete subscription
carries no trial_end, and writing it through would have erased the days the
account still had and locked it out.

Unpaid accounts are bounded by what they can cost us rather than by what they
can do: one workspace, one project, 3 GiB of direct uploads. YouTube imports,
share links, guests, comments and approvals stay unlimited, because those are
the parts worth trying and they cost nothing. isPaidTier() is the new seam;
hasBillingAccess() answers a different question now that access no longer
implies a card.

Signup CTAs, the pricing card, the comparison pages, the terms and the refund
policy all said the trial converts to a paid plan by itself. It no longer does,
so they say what happens instead. Settings and a banner name both dates that
matter: when the trial ends, and the fifteen days after that during which
nothing is deleted.

/admin/growth compares the two funnels on signup to paid within a fixed 30 day
window, not trial to paid. Dropping the card requirement multiplies trials, so
the old ratio can fall while more people actually pay, and reading it that way
would retire the change for the wrong reason.

lib/email-validation.ts

@@ -26,3 +26,73 @@ export function isValidEmailAddress(email: string): boolean {
 
   return labels.every((label) => label.length > 0 && label.length <= MAX_EMAIL_DOMAIN_LABEL_LENGTH);
 }
+
+/**
+ * Throwaway mailbox providers, refused at signup.
+ *
+ * The free trial is granted to any address somebody can read a link at, so a
+ * mailbox that costs nothing and expires in ten minutes is the cheapest way to
+ * take the trial repeatedly. This list is deliberately short and specific: it
+ * holds services whose entire purpose is a disposable inbox, and none of the
+ * forwarding or aliasing services (SimpleLogin, AnonAddy, Apple's Hide My Email,
+ * Fastmail masked addresses) that real paying customers use every day. A list
+ * that catches a genuine buyer costs far more than one that misses a scraper.
+ */
+const DISPOSABLE_EMAIL_DOMAINS = new Set([
+  '10minutemail.com',
+  'discard.email',
+  'dispostable.com',
+  'emailondeck.com',
+  'fakeinbox.com',
+  'getnada.com',
+  'grr.la',
+  'guerrillamail.com',
+  'guerrillamail.net',
+  'guerrillamail.org',
+  'harakirimail.com',
+  'inboxkitten.com',
+  'mailcatch.com',
+  'maildrop.cc',
+  'mailinator.com',
+  'mailnesia.com',
+  'mintemail.com',
+  'moakt.com',
+  'mohmal.com',
+  'nada.email',
+  'sharklasers.com',
+  'spam4.me',
+  'spamgourmet.com',
+  'temp-mail.org',
+  'tempinbox.com',
+  'tempmail.com',
+  'tempr.email',
+  'throwawaymail.com',
+  'tmpmail.org',
+  'trashmail.com',
+  'yopmail.com',
+  'yopmail.fr',
+  'yopmail.net',
+]);
+
+/**
+ * True when the address belongs to a known disposable mailbox provider.
+ *
+ * Parent domains are checked too, because several of these hand out per-visit
+ * subdomains (`anything.mailinator.com`) that would otherwise walk straight past
+ * an exact-match lookup.
+ */
+export function isDisposableEmailDomain(email: string): boolean {
+  const atIndex = email.lastIndexOf('@');
+  if (atIndex < 0) return false;
+
+  const domain = normalizeEmail(email.slice(atIndex + 1));
+  const labels = domain.split('.');
+
+  for (let index = 0; index < labels.length - 1; index += 1) {
+    if (DISPOSABLE_EMAIL_DOMAINS.has(labels.slice(index).join('.'))) {
+      return true;
+    }
+  }
+
+  return false;
+}