Retire the shared coturn service — every WebRTC/SIP service now runs its own

Shared coturn (services/coturn.sh, ensure_coturn_user in lib/common.sh) is no
longer an installable or usable option anywhere in this repo. It's moved to
attic/coturn.sh (with tools/coturn-test-check.sh alongside it), which is
outside setup.sh's services/*.sh glob, so it never registers, never appears
in the menu, and `sudo ./setup.sh coturn` now fails with "unknown service".

Asterisk and Mattermost each already had an opt-out to run their own
dedicated coturn instead of the shared one; that opt-out is now the only
behavior — the shared-coturn preference, the opt-out prompt, and every
ensure_coturn_user() call site are gone. find_free_coturn_range()
(lib/common.sh) is what makes unconditional dedicated coturn safe: it scans
every coturn-owning service's own .env on the box for already-claimed relay
ranges and picks one that can't collide, so Asterisk + any number of
Mattermost instances can each run their own coturn on one box without the
relay-port collisions this repo's coturn history warns about.

Existing installs still pointed at a shared coturn container are left
running as-is on `update` (no silent migration attempt against a service
that no longer exists to heal against) — a full/fresh reinstall is the
migration path, which generates a new dedicated coturn with fresh
credentials and says so.

Also updates CLAUDE.md's coturn guidance for future service authors,
attic/README.md with the retirement rationale, and stale
services/coturn.sh path references in services/asterisk.sh,
tools/pstn-test-check.sh, README.md, and docs/vps-sizing-recommendations.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Crt4ymNEHEbWqscB1qvZgC

docs/vps-sizing-recommendations.md

@@ -31,10 +31,16 @@ Rules of thumb:
   `ensure_swapfile()` unconditionally, which offers a 2GB swapfile any time
   RAM is ≤4096MB and none exists yet (`services/asterisk.sh` also calls it
   directly for the standalone-run case, so it's covered either way).
-- Sharing one `coturn` instance (`services/coturn.sh`) instead of letting
-  each WebRTC-capable service (Asterisk, Mattermost) embed its own saves a
-  container per consumer and — more importantly — avoids relay-port
-  collisions between them.
+- **Historical note, no longer applicable:** this doc's Tier 2/3 plans below
+  were sized around one shared `coturn` instance instead of each WebRTC-
+  capable service (Asterisk, Mattermost) embedding its own — it saved a
+  container per consumer and avoided relay-port collisions between them.
+  That shared-coturn service has since been retired from this repo (see
+  `attic/coturn.sh`); every service now runs its own dedicated coturn, and
+  `lib/common.sh`'s `find_free_coturn_range()` avoids the same relay-port
+  collisions by scanning each coturn-owning service's `.env` instead. Budget
+  a coturn container per WebRTC-capable service/instance, not one shared
+  ~40MB line, when re-planning a box from scratch.
 
 ## Tier 1 — ~1 vCPU / 1GB RAM / 25GB SSD
 
@@ -43,7 +49,7 @@ Example: DigitalOcean Basic, $6/mo.
 This is tight enough that Docker's own daemon overhead is already a
 meaningful fraction of the box. **Pick one purpose, not a stack:**
 
-- **Option A — Asterisk only.** Asterisk + the shared coturn service fits
+- **Option A — Asterisk only.** Asterisk + its own dedicated coturn fits
   comfortably per this repo's own droplet-sizing notes (`services/asterisk.sh`
   README section) — a swapfile is added automatically (RAM ≤4GB, see above),
   and this plan is "fine for a couple of extensions and light personal use."