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.

app/refund/page.tsx

@@ -47,13 +47,13 @@ export default function RefundPolicyPage() {
             <p>
               All new accounts are eligible for a{' '}
               <strong className="text-foreground">7-day free trial</strong> with full access to paid
-              features. We strongly encourage you to evaluate the Service fully during this period
-              before subscribing.
+              features. The trial requires no credit card and collects no payment. We strongly
+              encourage you to evaluate the Service fully during this period before subscribing.
             </p>
             <p className="mt-3">
-              You may cancel at any time during your free trial without being charged. If you do not
-              cancel before the trial ends, your chosen plan will automatically activate and payment
-              will be collected.
+              The trial never activates a paid plan by itself. When it ends, paid features stop
+              until you choose to subscribe, and a subscription is charged as soon as you complete
+              checkout.
             </p>
           </section>