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

commands/economy_support_commands.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import asyncio
 from collections.abc import Callable
 
 import discord
@@ -322,6 +323,46 @@ def register_economy_support_commands(
             msg = S.VANITY_UI["equipped"].format(emoji=v["emoji"], title=v["title"])
         await interaction.response.send_message(msg)
 
+    # -- /lootbox -----------------------------------------------------------
+    @tree.command(name="lootbox", description=S.CMD["lootbox"])
+    async def cmd_lootbox(interaction: discord.Interaction):
+        res = await economy.do_open_lootbox(interaction.user.id)
+        if not res["ok"]:
+            if res["reason"] == "db_error":
+                await reply_db_error(interaction)
+            elif res["reason"] == "banned":
+                await interaction.response.send_message(S.MSG_BANNED, ephemeral=True)
+            else:
+                await interaction.response.send_message(
+                    S.ERR["broke_need"].format(need=coin(res["need"])), ephemeral=True
+                )
+            return
+
+        # Suspense: show the box opening, then reveal the reward.
+        await interaction.response.send_message(
+            embed=discord.Embed(
+                title=S.LOOTBOX_UI["title"], description=S.LOOTBOX_UI["opening"], color=0xF4C430
+            )
+        )
+        msg = await interaction.original_response()
+        await asyncio.sleep(1.2)
+
+        if res["buff_kind"]:
+            line = S.LOOTBOX_UI["buff_" + res["buff_kind"]].format(min=res["buff_min"])
+            foot = S.LOOTBOX_UI["foot_buff"].format(balance=coin(res["balance"]))
+            color = 0x5865F2
+        else:
+            line = S.LOOTBOX_UI[res["outcome"]].format(coins=coin(res["reward_coins"]))
+            if res["net"] >= 0:
+                foot = S.LOOTBOX_UI["foot_win"].format(net=coin(res["net"]), balance=coin(res["balance"]))
+                color = 0x57F287
+            else:
+                foot = S.LOOTBOX_UI["foot_loss"].format(net=coin(abs(res["net"])), balance=coin(res["balance"]))
+                color = 0xF4C430 if res["outcome"] != "coins_small" else 0xED4245
+        embed = discord.Embed(title=S.LOOTBOX_UI["title"], description=line, color=color)
+        embed.set_footer(text=foot)
+        await msg.edit(embed=embed)
+
     class RemindersSelect(discord.ui.Select):
         def __init__(self, user_id: int, current: list[str]):
             self.user_id = user_id