Decouple internal SIP messaging management from PSTN trunk installation

Messaging has no dependency on a PSTN trunk existing (no cost, no
carrier, no DID), but the whole PSTN Trunk tab - including the messaging
checkbox added last commit - was hidden behind pstn_installed(), which
only becomes true once services/pstn-trunk.sh's dialplan is actually
wired in. That meant enabling messaging required going through full SIP
trunk/provider setup first for no real reason.

Added a standalone "Internal SIP messaging" card that's always visible in
the tab regardless of trunk status, backed by a new write_messaging()
that only touches the messaging key (leaving tier/allowed_numbers/
personal_did untouched) and creates pstn-permissions.conf from scratch if
it doesn't exist yet. Confirmed the systemd unit's ReadWritePaths and the
group/chmod grants already covered this - both are set up whenever a base
Asterisk install is detected, independent of pstn-trunk - so no
permission-layer changes were needed, only the dashboard's own artificial
UI gate.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ho9mZgAkVpdz7S5wJkg8Nf

services/pstn-trunk.sh

@@ -1817,9 +1817,10 @@ no carrier SMS, no PSTN, no cost) is gated by a \`messaging=yes\` flag per
 extension in \`pstn-permissions.conf\`, independent of the PSTN calling
 tiers above — off by default, same "opt in" posture. Currently enabled for:
 ${MESSAGING_EXTS:-none}. Live-editable any time via the Security
-Dashboard's "PSTN Trunk" tab (a checkbox per extension, right in the same
-table as the calling tiers) — no need to re-run this installer just to
-change who can message.
+Dashboard's "PSTN Trunk" tab, in its own always-available "Internal SIP
+messaging" card — no need to re-run this installer, and no dependency on
+this trunk (or any PSTN trunk at all) being installed, unlike the
+calling-permissions table below it in that same tab.
 
 **Won't show up in Easy Asterisk's own web admin, by design** — same as
 the PSTN calling tiers, this is a permission this repo layers on top,