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

test/server-core.test.js

@@ -35,7 +35,7 @@ test('client stores simadmin_session from login and sends it on later proxied re
   assert.equal(auth.authenticated, true)
   const proxied = await client.fetchJson('/api/device')
   assert.equal(proxied.status, 200)
-  assert.match(calls.at(-1).options.headers.cookie, /simadmin_session=abc123/)
+  assert.match(calls.at(-1).options.headers.get('cookie'), /simadmin_session=abc123/)
 })
 
 test('snapshot summary exposes device-monitoring fields from stats, sms, data and hardware endpoints', () => {