docs(decisions): record plan-tier taxonomy centralization decision (Option B)

Captures the 2026-05-29 decision to derive admin plan dropdown + validation
from the plan_limits table rather than hand-duplicating the allow-list across
6+ sites. Triggered by the prod "AI sessions down" report that traced to the
admin dropdown still offering the dead 'team' slug. Adds the matching backlog
entry to TODO.md with duplication sites enumerated.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.ai/TODO.md

@@ -23,3 +23,5 @@ None selected. Pick from the backlog below or `03-DEVELOPMENT-ROADMAP.md`.
 - [ ] **`bg-card-hover` Tailwind class doesn't resolve.** [`frontend/src/components/layout/CommandPalette.tsx:450-451`](../frontend/src/components/layout/CommandPalette.tsx) uses `bg-card-hover` as a Tailwind utility, but Tailwind v4 generates `bg-{token}` from `--color-{token}` — and the token in [`frontend/src/index.css:15`](../frontend/src/index.css) is `--color-bg-card-hover`, which generates `bg-bg-card-hover`, not `bg-card-hover`. So those classes silently produce nothing. Other call sites (KnowledgeBaseCards, TeamSummary, ProposalBanner) use the explicit `hover:bg-[var(--color-bg-card-hover)]` form which works. Fix: change the CommandPalette classes to the explicit-var form, OR add a `--color-card-hover` semantic mapping in index.css alongside `--color-card`. Surfaced 2026-05-01 during impeccable polish sweep.
 
 - [ ] **`ConcludeSessionModal` paused/escalated step forces single-artifact choice — should allow multi-select.** [`frontend/src/components/assistant/ConcludeSessionModal.tsx`](../frontend/src/components/assistant/ConcludeSessionModal.tsx) ~lines 430-474 ("Paused/Escalated: status update options"). Today the engineer clicks ONE of Ticket Notes / Client Update / Email Draft, the buttons disappear, and the result replaces them. Real MSP escalations almost always need at least two: technical notes for the next engineer's PSA AND a non-technical client update. Same for pause (client update + ticket notes for context when resuming). Recommended shape: multi-select with smart defaults — three checkboxes (`☑ Ticket Notes  ☑ Client Update  ☐ Email Draft`); for `escalated` pre-check Ticket Notes + Client Update; for `paused` pre-check Client Update only. One "Generate" button fires all selected in parallel via existing `aiSessionsApi.generateStatusUpdate(...)` (already supports the three `audience` values: `ticket_notes`, `client_update`, `email_draft`). Each result renders in its own card with its own Copy / Post-to-PSA / Send-Email action. Surfaced 2026-05-01. Feature work, not polish — touches streaming wiring for parallel calls.
+
+- [ ] **Centralize plan-tier taxonomy — derive admin plan dropdown (and validation) from `plan_limits`, not hardcoded lists.** Chose **Option B** over a one-line patch (see [DECISIONS.md](DECISIONS.md) 2026-05-29). *Surfaced by a prod bug (2026-05-28):* the admin "Change Plan" dropdown at [`AccountDetailPage.tsx:443-445`](../frontend/src/pages/admin/AccountDetailPage.tsx) still offered `free / pro / team` — the dead `team` slug (renamed to `enterprise` in migration `4ce3e594cb87`, 2026-05-07) and missing `starter`/`enterprise`. Selecting "Team" sends `{plan:"team"}` to `PUT /admin/accounts/{id}/subscription/plan`, which 400s on `if data.plan not in ("free","pro","starter","enterprise")` ([admin.py:994](../backend/app/api/endpoints/admin.py#L994), duplicated at [:975](../backend/app/api/endpoints/admin.py#L975)). The 400 detail was swallowed by a generic `toast.error('Failed to update plan')` ([AccountDetailPage.tsx:196](../frontend/src/pages/admin/AccountDetailPage.tsx)), so it presented as "AI sessions are down" (real cause: owner account had no paid plan; AI is plan-gated). **Root cause of the root cause:** the allowed-plan list is hand-duplicated across ≥6 sites and drifted (2nd such incident). **Duplication sites to consolidate:** backend [`admin.py:975`](../backend/app/api/endpoints/admin.py#L975) + [`:994`](../backend/app/api/endpoints/admin.py#L994) (tuple, twice), [`schemas/admin.py:128`](../backend/app/schemas/admin.py) (`AdminAccountCreate.plan` Literal), frontend `AccountDetailPage.tsx` dropdown, `AccountsPage.tsx` create-account dropdown, `types/admin.ts` + `types/account.ts` + `types/billing.ts`, `hooks/useSubscription.ts` (`isPaidPlan`), `components/subscription/CheckoutButton.tsx` (`planLabels`). **Source of truth:** the `plan_limits` table (rows: free/starter/pro/enterprise) — `PlanLimitWithBillingResponse` already exposes `is_public` + `sort_order` + `display_name` for ordering/labels. **End state (B):** admin dropdown + pricing/checkout derive options from a plans endpoint backed by `plan_limits` (filter `is_public`, order by `sort_order`, label from `display_name`); backend validation checks against actual `plan_limits` rows instead of a hardcoded tuple. **Trivial first commit (land anytime to unblock the admin tool):** fix the `AccountDetailPage` dropdown to `Free / Starter / Pro / Enterprise` and surface the backend error detail in the toast. ⚠️ The `'team'` string in `Tree.visibility` / `StepLibrary.visibility` is a *separate domain* (shared-with-account) — do NOT touch it.