# Seat Management

## Objective

Show which members cost money and stop surprise overages.

Seat accounting that defines which member states are billable, enforces the limit at invite time, and stays reconciled with the billing provider.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Define one rule for what consumes a seat — which of active, pending, suspended, deactivated, guest, and service accounts count — and apply that single rule everywhere the number is shown or enforced.
2. Show used, total, and remaining seats wherever members are managed, and state the rule in plain words next to it. A number nobody can predict generates support tickets.
3. Enforce the limit at the point of invitation and again at the point of acceptance. Checking only at invite time lets a batch of pending invites overshoot the plan.
4. When the limit is reached, either block with a clear upgrade path or queue the invite until a seat frees. Pick one behaviour and make it explicit — do not let the invite half-succeed.
5. Reconcile app membership against the billing provider's subscription quantity on a schedule, and alert when they drift. Webhooks are missed and API calls fail; drift is guaranteed over time.
6. Do NOT let a seat reduction remove a member who is required for the organization to function, such as its last owner. Refuse the downgrade and say why.
7. Proration arithmetic and its preview are owned by Proration Preview; call into that rather than computing money here.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- Removing and re-adding the same person within a billing period must not be charged as two seats.
- A suspended member must follow the stated rule consistently — if suspension frees a seat, reactivation must be blocked when the seats are gone, and the user must be told before they suspend.
- Two invites accepted at the same instant must not both take the last seat; enforce the limit with a database-level constraint, not an application read-then-write.
- A downgrade below current usage must be resolved before it takes effect — the admin chooses who to remove, rather than the system silently locking people out.
- Expired or revoked pending invitations must release their seat immediately, not at the next billing cycle.
- A failed payment or cancelled subscription needs a defined seat state; falling back to unlimited is a revenue leak and falling back to zero locks out paying customers.
- Where the billing provider disagrees with the app, decide which side is authoritative and repair in one direction only.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] One documented rule defines which member states consume a seat, and every count in the app agrees with it.
- [ ] Used, total, and remaining seats are visible wherever members are managed.
- [ ] The limit is enforced at both invitation and acceptance, at the database level.
- [ ] Reaching the limit produces a defined block or queue with an upgrade path, never a partial invite.
- [ ] App membership is reconciled against the billing provider's quantity on a schedule, with alerts on drift.
- [ ] A downgrade cannot remove the last owner or silently deactivate members.
- [ ] Removing and re-adding a member within a period does not double-charge.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.