feat(event-triage): per-user check interval, overriding the instance one

Event triage is the one system agent whose right cadence depends on who it
runs for: it fires on inbound events, so someone on a dozen mailing lists
has something waiting on nearly every tick while a quiet account has
something waiting almost never. A single instance-wide interval serves one
of them badly, and the observed failure is the first: the agent starts on
practically every pass.

An admin can now set a per-person interval on that user's page (Users ->
the person -> Event triage). Empty means "follow the instance setting",
which stays the state nobody has a row for.

- New registry table `system_agent_user_settings(agent_id, user_id,
  interval_secs)`. A row is an override and its absence is inheritance --
  no sentinel value, no row seeded at user creation, clearing the field
  deletes the row. Registry rather than the user's own file because the
  writer is the admin and a member's database is unreadable unless they
  happen to be logged in; a setting that could only be changed during its
  subject's session would not be a setting. Keyed by agent_id though only
  one agent uses it, so a future agent's schedule is not a schema change.

- `SystemAgent` gains `interval_secs_for(user_id)`, which `is_due` now
  measures against, and `shortest_interval_secs()`. Both default to the
  existing `interval_secs`, so every other agent implements nothing. The
  second is the non-obvious half: `base_tick` sleeps for the shortest
  interval any enabled agent asks for, so without it an override below the
  instance value would be rounded up to it -- an override that works when
  it lengthens and silently does nothing when it shortens.

- `GET/PUT /api/users/{id}/event-triage`, admin-gated, minutes on the
  wire, null to clear. Nothing rides the bus: the scheduler re-reads the
  interval every tick and due-ness is counted from the user's own last
  attempt, so a change lands on the next wake-up with no push.

Both helpers fail open onto the instance value -- an unreadable registry
must not turn into an agent that stops running for someone.

Docs: docs/system-agents.md gains the per-person section and no longer
reads as if the interval were one number for everybody.

web/i18n/it.js

@@ -1099,6 +1099,7 @@ export default {
   'users.detail.profile':              'Profilo',
   'users.detail.connectors':           'Connettori',
   'users.detail.plugins':              'Plugin',
+  'users.detail.triage':               'Triage eventi',
   'users.detail.security':             'Sicurezza',
   'users.detail.saved':                'Salvato',
   'users.detail.delete_hint':          'Elimina questo utente, il suo database e tutta la cronologia delle conversazioni.',
@@ -1115,6 +1116,13 @@ export default {
   'users.plug.empty':                  'Nessun plugin disponibile.',
   'users.plug.admin_note':             'Gli amministratori possono usare qualsiasi plugin abilitato, indipendentemente da ciò che è selezionato qui.',
 
+  'users.triage.hint':                 'Il triage eventi legge gli eventi arrivati dai connettori di questa persona e le segnala quelli che meritano un\'interruzione. Chi riceve molta posta o molti messaggi lo fa partire quasi a ogni passaggio: qui puoi dargli una cadenza più lenta.',
+  'users.triage.interval':             'Intervallo di controllo (minuti)',
+  'users.triage.placeholder':          'Predefinito ({n})',
+  'users.triage.using_default':        'Vuoto: segue l\'impostazione dell\'istanza ({n} min).',
+  'users.triage.using_override':       'Solo per questo utente. Svuota il campo per tornare all\'impostazione dell\'istanza.',
+  'users.triage.invalid':              'Inserisci un numero intero di minuti, oppure lascia il campo vuoto.',
+
   'users.modal.create_title':          'Nuovo utente',
   'users.modal.username':              'Nome utente',
   'users.modal.display_name':          'Nome visualizzato',