feat(billing): enforce free-plan student cap across the app (#733)

* feat(billing): enforce free-plan student cap across the app

Free (BASIC) plan orgs were able to exceed the advertised 20-student
limit because the cap existed only as dashboard cosmetic copy with no
server-side enforcement.

Add a single source of truth (getStudentLimit / STUDENT_LIMITS) and a
central guard (assertStudentCapacityOrThrow) wired into every path that
creates a new student-role org member: self-enrolment, course-invite
acceptance, bulk audience import, and org-invite acceptance. The guard
throws UPGRADE_REQUIRED (403) and notifies org admins by email (throttled
to once per org per day) when the limit is hit.

Surface the limit in the UI: disable the audience import button and the
course new-student invite section with upgrade banners for admins, and
show a "contact your administrator" message on the public enrol page for
students (who cannot upgrade). Expose a studentLimitReached flag from the
public course endpoint so the enrol button can be disabled proactively.

Add a one-time (manual, non-cron) db script,
db:notify-students-over-limit, to email admins of orgs already over the
limit at rollout.

* feat(billing): guard remaining student-creating paths against the cap

The initial cap enforcement missed several service entry points that also
create a new student-role org member. Add the capacity guard to:

- autoEnrollStudent (public org self-join on a tenant site)
- cohort enrollment (bulk group enrollment and single member accept)
- ensureProgramCourseAccess (program/cohort-granted course access)

For the cohort and program paths, only genuinely new members are counted
(existing org members are skipped) so re-enrolling current students is
never falsely blocked. The core path enforces via the shared limit check;
it throws UPGRADE_REQUIRED without the admin email since the queue lives
in the api layer.

* fix(billing): don't show cap block to existing org members on enroll page

The public course-by-slug endpoint is unauthenticated, so the enroll
page's studentLimitReached flag is org-wide and can't tell whether the
current viewer is already an org member. An existing student of a full
BASIC org would see Join disabled even though the backend correctly lets
existing members enroll without consuming a new seat.

Only pre-block anonymous visitors (would-be new joiners). Logged-in users
are governed by the membership-aware backend, which allows existing
members and returns UPGRADE_REQUIRED (surfaced via the enroll onError
mapping) for genuinely new students.

* feat(billing): deliver org plan-limits via /account with an extensible registry

Replace the ad-hoc, per-surface audience fetches (each calling getOrgAudience
just to read a count) with a single, future-proof delivery of org usage +
limits on the /account payload, gated to admin/tutor members (students never
receive limit data; a plain COUNT, no student rows).

Model it generically so future org limits (courses, etc.) drop in without
reshaping the payload or re-plumbing the frontend:
- New packages/utils plan-limits registry: PLAN_LIMIT_RESOURCES, PLAN_LIMITS
  (reuses STUDENT_LIMITS), getPlanLimit, and a JSON-safe ResourceUsage /
  OrgUsageLimits shape with isResourceLimitReached / resourceLimitRemaining.
- getAccountData attaches a per-org `limits` map for admin/tutor orgs.
- Store selectors currentOrgLimits / studentUsage / isStudentLimitReached;
  sidebars, upgrade card, People pages, and invite modals read these instead
  of fetching audience rows.

Bring cohort surfaces to parity with course: cohort People page banner and
cohort sidebar People upgrade icon, matching the course equivalents.

Also: close the upgrade modal that opened over the invite modal (drop the
`add` param via openUpgradeModalOver), redesign the sidebar upgrade card
(progress toward the student limit), add a Progress `variant`, and document
the no-complex-expressions-in-object-literals rule in AGENTS.md.

* refactor(billing): address PR review on the limits work

- Dedupe new student profile IDs before the cohort seat check so duplicate
  batch entries can't overcount against the cap (Greptile P1).
- Drive the org-sidebar upgrade indicator from a dynamic nav-item `upgrade`
  flag (audience config declares `upgradeResource: 'students'`; groups set it
  from a `limitsReached` map) instead of a hardcoded path check.
- Collapse the audience Import button into a single button with ternary
  `disabled`/`href`.
- Simplify the notify-students-over-limit upgrade link to app.classroomio.com.
- Drop the comment on the org `limits` field.
- Add .greptile/rules.md documenting the manual one-time db/src/scripts
  convention (operator-controlled reruns) so idempotency isn't reflagged.

* fix(billing): name the org in the enroll capacity message

Change the enroll-page student-limit message from "This course has reached
its student capacity" to "{orgName} has reached its student capacity", since
the cap is org-wide, not course-specific. Pass orgName from the loaded org on
the page and from the currentOrg store in the enroll onError snackbar.

* style(billing): use PremiumIcon for the Authentication upgrade indicator

Replace the RocketIcon on paid settings sub-items (e.g. Authentication) with
the PremiumIcon used on the Audience item, right-aligned (ml-auto) to match.

packages/utils/src/plans/constants.ts

@@ -62,3 +62,17 @@ export function getWorkspaceAllowance(planName: string | null | undefined): numb
 export function canCreateWorkspaces(planName: string | null | undefined): boolean {
   return planName === PLAN.ENTERPRISE;
 }
+
+/**
+ * Max students allowed per plan (org-wide, counts ROLE.STUDENT organizationmember rows).
+ */
+export const STUDENT_LIMITS: Record<string, number> = {
+  [PLAN.BASIC]: 20,
+  [PLAN.EARLY_ADOPTER]: 10000,
+  [PLAN.ENTERPRISE]: Infinity
+};
+
+export function getStudentLimit(planName: string | null | undefined): number {
+  if (!planName) return STUDENT_LIMITS[PLAN.BASIC];
+  return STUDENT_LIMITS[planName] ?? STUDENT_LIMITS[PLAN.BASIC];
+}

packages/utils/src/plans/index.ts

@@ -1,4 +1,5 @@
 export * from './constants';
+export * from './limits';
 export * from './automation';
 export * from './public-api';
 export * from './org-plan';

packages/utils/src/plans/limits.ts

@@ -0,0 +1,41 @@
+import { PLAN, STUDENT_LIMITS } from './constants';
+
+/**
+ * Org-scoped resources that carry a per-plan limit and are surfaced to the
+ * dashboard. Extend this tuple (plus PLAN_LIMITS) to add a new limited resource.
+ */
+export const PLAN_LIMIT_RESOURCES = ['students'] as const;
+export type PlanLimitResource = (typeof PLAN_LIMIT_RESOURCES)[number];
+
+/** Per-resource, per-plan numeric caps (Infinity = unlimited). */
+export const PLAN_LIMITS: Record<PlanLimitResource, Record<string, number>> = {
+  students: STUDENT_LIMITS
+};
+
+export function getPlanLimit(resource: PlanLimitResource, planName: string | null | undefined): number {
+  const table = PLAN_LIMITS[resource];
+  if (!planName) return table[PLAN.BASIC];
+
+  return table[planName] ?? table[PLAN.BASIC];
+}
+
+/** JSON-safe usage record carried in the account payload. `limit: null` = unlimited. */
+export type ResourceUsage = { used: number; limit: number | null };
+export type OrgUsageLimits = Partial<Record<PlanLimitResource, ResourceUsage>>;
+
+/** Build a usage record, converting a non-JSON-safe Infinity limit to null. */
+export function toResourceUsage(used: number, planLimit: number): ResourceUsage {
+  return { used, limit: Number.isFinite(planLimit) ? planLimit : null };
+}
+
+export function isResourceLimitReached(usage: ResourceUsage | undefined): boolean {
+  if (!usage || usage.limit === null) return false;
+
+  return usage.used >= usage.limit;
+}
+
+export function resourceLimitRemaining(usage: ResourceUsage | undefined): number | null {
+  if (!usage || usage.limit === null) return null;
+
+  return Math.max(0, usage.limit - usage.used);
+}