Add email channel: IMAP IDLE inbound, SMTP outbound, thread matching

Inbound mail on support@top-sysops.ru creates/updates tickets in realtime
(IMAP IDLE, not polling). Replies thread via In-Reply-To/References against
stored Message-IDs, falling back to the customer's open ticket. Mailbox
credentials configured and encrypted via Settings -> Почта, same pattern as
the Telegram channel. TLS verification is opt-in-skippable per mailbox
(mail.top-sysops.ru's cert is currently expired; LAN-only server).

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

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

@@ -8,6 +8,13 @@
       "when": 1785068349476,
       "tag": "0000_superb_the_fallen",
       "breakpoints": true
+    },
+    {
+      "idx": 1,
+      "version": "6",
+      "when": 1785070407727,
+      "tag": "0001_mushy_purple_man",
+      "breakpoints": true
     }
   ]
 }