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

core/economy/store.py

@@ -152,6 +152,7 @@ class UserData(TypedDict, total=False):
     total_given: int
     total_received: int
     best_daily_streak: int
+    lootboxes_opened: int
     heist_global_cd_until: float
     # Prestige system
     prestige_level: int
@@ -213,6 +214,7 @@ def _default_user() -> UserData:
         "total_given":     0,
         "total_received":  0,
         "best_daily_streak": 0,
+        "lootboxes_opened": 0,
         "heist_global_cd_until": 0.0,
         # ── Prestige ─────────────────────────────────────────────────────────
         "prestige_level":    0,