Add "Sent with SendLit" badge to emails sent on the Free plan #57

Background

The proposed pricing has the Free plan (1 team, 3k sends/month) carry a small "Sent with SendLit" footer, while paid plans (Pro, Business, Enterprise) send without it. A badge on free sends turns every email into a referral to app.sendlit.app. It also gives people a clear reason to upgrade, which fits how we want to position SendLit: you pay for sends, not for removing branding. (Beehiiv keeps its badge even on paid plans.) No badge code exists today.

Plan entitlements live in the billing work (PR #16: billing/policies.ts, entitlements.ts), so this depends on that landing.

Scope

• Add a brandingRequired (or similar) entitlement, true for Free and false for paid plans.
• At send time, add a small footer to marketing emails (broadcasts and sequences) in both the HTML and plain-text parts: "Sent with SendLit", linking to the SendLit site with UTM params.
• Transactional/API sends: decide whether they get the badge (default: no).
• The badge goes away as soon as a team upgrades, and comes back if it downgrades.

Acceptance criteria

• Free-plan marketing emails include the footer in both the HTML and plain-text bodies.
• Pro/Business/Enterprise emails never include it.
• Teams can't remove the footer through templates or the editor while on Free.
• A plan change takes effect on the next send.
• Unit tests cover entitlement resolution and footer injection.
• Docs and the pricing page mention the badge on Free.