Credit packs and admin panel

What's new

• Credit packs — organisation owners and admins can top up with one-time credit purchases from the billing page.
• Admin panel — platform administrators get a dedicated area at /admin to manage every user and organisation.
• Impersonation — admins can act as any user for debugging or support. The impersonated session shows a banner so nobody forgets it's on.

Improvements

• Per-seat billing is now a config option on any Plan (priceModel: "per_seat").
• Webhooks send transactional emails on payment failure, subscription cancellation, disputes, and trial-ending-soon notices.

Fixes

• Expired bans auto-clear on the next session check instead of requiring a manual unban.