fix: apply security and correctness review findings across both stacks

Legacy panel:
- upstream body reads now carry their own deadline and a 10 MB byte budget;
  a stalled modem can no longer hang /api/status fan-out forever nor OOM
  the proxy (request headers alone had the timeout, bodies had none)
- add X-Frame-Options DENY / CSP frame-ancestors none / nosniff; the panel
  (delete-instance and confirmed-write dialogs) is no longer clickjackable
- only send a JSON content-type when the API console request has a body, so
  payload-less dangerous writes stop failing with 400 and burning the
  one-use confirmation token
- register a form-urlencoded parser (the proxy branch was unreachable) and
  drop the multipart parser that buffered up to 60 MB before rejecting;
  bodyLimit drops to 2 MB; framework-level 415 keeps the stable error body
- remove /api/sms/send from readable paths: GET bypassed the write
  confirmation for a send endpoint
- /api/instances/:id/login maps upstream failures to a stable 502 instead
  of leaking raw error text
- guard MULTI_SIMADMIN_TIMEOUT_MS parsing (NaN aborted every request);
  prune dead code (buildClients, cookie expando no-op)

Control plane:
- deleting a notification channel detaches it from rules instead of leaving
  dangling ids that made every referencing rule unreadable and silently
  dropped future notifications; rule reads tolerate unknown ids
- startup sweep resets notification_queue rows stranded in 'sending' by a
  crash (mirrors the sms outbox sweep); terminal outbox rows are pruned on
  the retention timer
- /api/v1/metrics no longer emits operator-assigned node names on the
  session-free scrape; login limiter map is bounded and pruned; Secure
  cookie honors the gateway-declared x-forwarded-proto
- webhook delivery sets redirect: manual (signed payloads are not replayed)
- SMTP envelope sender is validated against CR/LF smuggling
- scheduled reboots with delaySeconds != 3 fail fast at the dispatcher with
  a clear reason instead of burning every retry; contract narrowed to the
  pinned baseline

packages/contracts/src/automation.ts

@@ -417,10 +417,12 @@ export function parseCreateScheduledTaskRequest(value: unknown): CreateScheduled
     throw new TypeError('effectiveEndAt must be later than effectiveStartAt');
   if (source.enabled !== undefined && typeof source.enabled !== 'boolean')
     throw new TypeError('enabled must be boolean');
+  // The upstream execution baseline pins reboot delay_seconds to 3; accepting a
+  // wider range here would only produce tasks that can never run.
   const delaySeconds =
     source.delaySeconds === undefined
       ? undefined
-      : numberInRange(source.delaySeconds, 'delaySeconds', 0, 3_600);
+      : numberInRange(source.delaySeconds, 'delaySeconds', 3, 3);
   if (delaySeconds !== undefined && operationType !== 'reboot-system')
     throw new TypeError('delaySeconds is only valid for reboot-system');
   return {