paging: paging groups, *80 intercom, auto-answer headers

/api/paging-groups CRUD (members as memberIds). The generator renders
sp-page-{id} per 03 §6.11 (desk phones only, auto-answer headers via
sp-sub-autoanswer) plus the number in sp-local-numbers. Added the `s`
option so phones already on a call aren't paged, which also keeps a
pager who is a member from being paged themselves. A group with no
members just hangs up.

*80<ext> intercom checks the target first: digits only and an endpoint
in sp-from-ext, else ss-noservice (the spec's one-liner passed
${EXTEN:3} straight into Page()). Both written back into 03.

Web: Paging page (list, drawer form with member checkboxes). Golden
case paging. e2e m8-t4 with baresip phones that honor the headers
(sip_autoanswer): an ordinary call rings, a page is answered by itself
and muted, the busy pager is skipped, *80 is two-way, *80999 refused.
The check on a real Yealink/Grandstream stays open (09).

Task: M8-T4

docs/spec/09-roadmap.md

@@ -205,6 +205,7 @@ Legend: 📖 read · 🔨 build · ✅ done when
 **M8-T2** Parking (`res_parking.conf`, `sp-parkedcalls` include) + parked calls in the live UI + BLF hints for slots.
 **M8-T3** Conferences with PIN/admin PIN (03 §6.11).
 **M8-T4** Paging/intercom (`sp-page-*`, `*80`) + auto-answer test on a real Yealink or Grandstream.
+As built: tested with baresip phones that honor the auto-answer headers (`sip_autoanswer`). **Open:** the check on a real Yealink or Grandstream, when one is available.
 **M8-T5** MOH classes (03 §8).
 ✅ Golden `queue` case passes. Scenario: 3 callers wait in a queue with 2 agents. One agent pauses with `*45550`. The caller who waits past `max_wait_seconds` goes to the timeout destination. A parked call on 701 can be picked up from another phone.