Add embeddable chat widget for websites

Reuses the existing portal-token/SSE mechanism (customers.portalToken,
/api/portal/events) for anonymous widget visitors instead of building a
parallel auth system. Public API: session bootstrap + messages endpoint,
both scoped by a non-secret siteKey with optional origin allowlist.
Embeddable script (public/widget.js) is a small self-contained vanilla JS
file: floating button + iframe kept mounted for a live SSE connection,
with a postMessage bridge for the unread badge. Admin UI at
Settings -> Виджет manages sites and shows the embed snippet.

Also generalized recordTelegramInboundMessage -> recordChatInboundMessage
(channel param) since Telegram and the widget need the identical
find-or-create-open-ticket heuristic.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QcXH24ky6zjk2UyK5oZUPH

src/lib/db/migrations/meta/_journal.json

@@ -15,6 +15,13 @@
       "when": 1785070407727,
       "tag": "0001_mushy_purple_man",
       "breakpoints": true
+    },
+    {
+      "idx": 2,
+      "version": "6",
+      "when": 1785072485453,
+      "tag": "0002_broad_stellaris",
+      "breakpoints": true
     }
   ]
 }