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/economy.py

@@ -18,6 +18,7 @@ __all__ = [
     'CONSUMABLES_UI',
     'CONSUMABLE_DESCRIPTIONS',
     'VANITY_UI',
+    'LOOTBOX_UI',
     'JAILED_UI',
     'SHOP_BTN',
     'DAILY_UI',
@@ -257,6 +258,23 @@ VANITY_UI: dict[str, str] = {
     "unequipped":  "Märk eemaldatud - su profiil on jälle tavaline.",
 }
 
+# ---------------------------------------------------------------------------
+# Lootbox  (pay-to-open mystery box, a coin sink with a random reward)
+# ---------------------------------------------------------------------------
+LOOTBOX_UI: dict[str, str] = {
+    "title":         "🎁 Õnnekast",
+    "opening":       "🎁 Avan õnnekasti...",
+    "coins_small":   "🪙 Leidsid põhjast paar münti: **+{coins}**",
+    "coins_medium":  "💰 Korralik saak: **+{coins}**",
+    "coins_big":     "💎 Suur õnn: **+{coins}**",
+    "jackpot":       "🎉 **JACKPOT!** **+{coins}**",
+    "buff_earn":     "⚡ Boonus: teenimine **×2** järgmiseks **{min} minutiks**!",
+    "buff_exp":      "✨ Boonus: EXP **×2** järgmiseks **{min} minutiks**!",
+    "foot_win":      "Netovõit: +{net}  ·  Saldo: {balance}",
+    "foot_loss":     "Netokahjum: {net}  ·  Saldo: {balance}",
+    "foot_buff":     "Saldo: {balance}",
+}
+
 JAILED_UI: dict[str, str] = {
     "title":      "🔒 Praegu vanglas",
     "empty":      "Kõik on vabad! Vanglas pole kedagi.",