feat(errors): per-account error modal with persisted error list

Accounts finishing done_with_errors showed only a count and a single
last_error. This adds a modal listing every concrete error of the
account's most recent run.

- migration 0005: account_errors table (kind folder|message|account,
  folder, message_ref, error, created_at; ON DELETE CASCADE; indexed)
- store: AddAccountError / ClearAccountErrors / ListAccountErrors
- copy: OnError callback captures per-message error text (previously
  only counted), with a "UID N: subject" reference
- orchestrator: clear errors at run start; persist folder/message/
  account errors; cap 500 rows/account/run with a suppressed-note row
- api: GET /api/tasks/{id}/accounts/{accountId}/errors
- web: AccountErrorsModal, clickable ERRORS count, api + styles

Verified: migration applies on Postgres 18; store add/list/clear and
cascade tests pass against real pg; backend build/vet/test green; web
tsc+vite build and oxlint clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U9Eq4JtWjyNTv5qat3B3mM

web/src/api.ts

@@ -157,6 +157,20 @@ export const setTaskSchedule = (taskId: number, intervalSeconds: number) =>
 
 export const listRuns = (taskId: number) => api<Run[]>(`/api/tasks/${taskId}/runs`)
 
+export interface AccountError {
+  id: number
+  account_id: number
+  run_id: number
+  kind: string // folder | message | account
+  folder: string
+  message_ref: string
+  error: string
+  created_at: string
+}
+
+export const listAccountErrors = (taskId: number, accountId: number) =>
+  api<AccountError[]>(`/api/tasks/${taskId}/accounts/${accountId}/errors`)
+
 export const importCSV = (id: number, file: File) => {
   const fd = new FormData()
   fd.append('file', file)