feat: daily reminder mail for erroneous tasks (admins + WF owners, per-user opt-out)

[JeJan]

What

Daily e-mail digest of all workflow tasks in an error state (see ADR-009):

• Admins (role wf-admin + EMAIL_RECEIVERS_ERRONEOUS_TASKS) receive a global overview across all workflows; WF owners (BPMN property wf-owner) only receive the errors of their own workflows. Users who are both get the admin mail only.
• Errors that are new since the last digest are marked with * NEW *; each entry shows since when the task has been erroneous (new column error_at).
• Per-user opt-out via the user settings (new switch, default: on). The static receiver list is exempt from the opt-out.
• No mail is sent when there is nothing to report (for the respective recipient); tasks of cancelled/completed instances are no longer reported.

Configuration

• EMAIL_ERRONEOUS_TASKS_REMINDER_CRON (default 0 7 * * *, empty = disabled; invalid expressions are logged and disable only this job)
• EMAIL_RECEIVERS_ERRONEOUS_TASKS (existing list, always receives the global digest)

Migration

e8b3d5c7a9f1: workflow_instance_tasks.error_at + error_reported_at, workflow_users.receive_error_task_reminder (default on). Runs on app startup as usual.

Tests

10 new tests (test_erroneous_reminder.py): new/known cycle, recovery reset, owner scoping, admin dedup, opt-out incl. static-list exemption, partial send failures, cancelled instances. Full backend suite green, frontend typecheck green. Verified end-to-end against a local SMTP catcher (Mailpit).

Note

The migration shares its parent (d4f7a1e9c2b6) with the one from the form-template PR — whoever merges second must rebase their migration or create an Alembic merge revision.