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

attic/README.md

@@ -34,3 +34,33 @@ rather than `fresh` at the reinstall prompt, and having a snapshot.
 Two copies of the same logic is the exact problem the merge existed to fix,
 and this one will drift the moment `services/asterisk.sh` gets a fix that
 isn't backported here — which it deliberately won't be.
+
+## `coturn.sh` / `coturn-test-check.sh`
+
+The shared-coturn service (`services/coturn.sh`, moved here unchanged from
+`services/`) and its standalone health-check tool (`tools/coturn-test-check.sh`,
+moved from `tools/`). This model — every WebRTC/SIP-capable service
+(`asterisk`, `mattermost`) sharing one coturn instance via `ensure_coturn_user`
+— is no longer offered anywhere in this repo. Every service now runs its own
+dedicated coturn instead, with `lib/common.sh`'s `find_free_coturn_range()`
+avoiding the relay-port collisions a shared instance used to prevent by
+scanning every coturn-owning service's own `.env` on the box. See
+`CLAUDE.md`'s "coturn (TURN/STUN) relay" section for the current pattern.
+
+Sharing one instance only ever saved ~40MB RAM per additional consumer
+beyond the first — real, but small — against being a single point of
+failure every consumer depended on. Parked here, not deleted, since the
+code is still correct and someone could resurrect it if a future need for
+it shows up. Both files still run standalone if invoked directly:
+
+```bash
+sudo bash attic/coturn.sh
+sudo bash attic/coturn-test-check.sh
+```
+
+Nothing in this repo calls `ensure_coturn_user()` anymore (the function
+itself was removed from `lib/common.sh`), so resurrecting this only makes
+sense if you're deliberately reintroducing the shared-coturn pattern
+yourself — a new consumer service would need its own call to whatever
+takes `ensure_coturn_user`'s place, since that helper no longer exists to
+call.