feat(economy): add /lootbox mystery box (coin sink with random reward)

A pay-to-open box (1000 coins) with a weighted reward: coin tiers (usually a net
loss - the sink), a random 30-min earn/exp buff, or a rare jackpot. All rolling
lives in do_open_lootbox for testability; the command adds a short reveal.

- New core module lootbox.py; extracted consumables.grant_buff (reused by both
  consumables and lootbox) to avoid duplicating the buff-stacking logic.
- New lootboxes_opened stat; pending schema syncs as a number automatically.
- Added /consumables, /lootbox, /vanity to the help embed (the first two were
  previously missing from /help).

Tests cover charging, insufficient/banned, the coin and buff outcomes, the
net-vs-reward invariant, and that balance never goes negative.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013VbAVsrZuYesea99mPMmPT

strings/__init__.py

@@ -59,6 +59,7 @@ from .economy import (
     CONSUMABLES_UI,
     CONSUMABLE_DESCRIPTIONS,
     VANITY_UI,
+    LOOTBOX_UI,
     JAILED_UI,
     SHOP_BTN,
     DAILY_UI,
@@ -152,6 +153,7 @@ __all__ = [
     'CONSUMABLES_UI',
     'CONSUMABLE_DESCRIPTIONS',
     'VANITY_UI',
+    'LOOTBOX_UI',
     'JAILED_UI',
     'SHOP_BTN',
     'DAILY_UI',