feat: meter AI usage

docs/architecture/ai-career-assistant.md

@@ -55,10 +55,13 @@ master profile text ────────────┘        │
 overwritten. This is deliberately distinct from `AiWorkspaceNote` (a one-row-per-type *cache* for
 candidate-fit/focus-plan). History gives the user restore/reuse (re-surface a past result), compare
 (view two side by side), copy, and delete. `ResultJson` is `{ text, meta? }`; `Provider` records which
-provider produced it. Tenant-scoped (owner query filter), cascades with the application.
+provider produced it. Each row also stores input/output character counts and a conservative estimated
+token count (characters ÷ 4); the estimate is provider-neutral because the sidecar currently returns
+text rather than provider billing metadata. Tenant-scoped (owner query filter), cascades with the application.
 
 API (`AiWorkspaceController`, `/api/jobapplications/{id}/ai`): `GET modules` (+ current provider),
-`POST generate`, `GET history?module=`, `DELETE history/{id}`.
+`POST generate`, `GET history?module=`, `DELETE history/{id}`. `GET /api/ai/usage` returns current-month
+and all-time totals; the workspace displays the monthly calls and estimated tokens.
 
 ## Provider abstraction