Rewrite guest confirmation text; add discount and multi-day checkout

Copy button text now matches the exact guest-facing template
requested — "Ваше бронирование подтверждено!", Гость/Номер для
связи/Объект бронирования/Количество гостей/Дополнительные
пожелания/Скидка — dropping the internal-only fields (source, who
booked it) since this text gets pasted straight to the client, not
kept for internal records (those still show on the on-screen summary
card).

New checkOutDate column (nullable — null means same-day, the common
case) lets a booking span multiple days; the Google Calendar link and
copy text both show the date range when set. New discount column
(same shape as prepayment) feeds the new "Скидка" line.

Statistics page gains a money row for the selected month: total
booked, prepayments collected, amount still outstanding, and total
discount given.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gh2UXUQUVBWroWEnn1FLFG

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

@@ -22,6 +22,13 @@
       "when": 1788004461991,
       "tag": "0002_plain_frightful_four",
       "breakpoints": true
+    },
+    {
+      "idx": 3,
+      "version": "6",
+      "when": 1789328671133,
+      "tag": "0003_little_bishop",
+      "breakpoints": true
     }
   ]
 }