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/commands.py

@@ -75,6 +75,7 @@ CMD: dict[str, str] = {
     "quests":         "Vaata oma päeva- ja nädalaülesandeid ning nõua auhinnad",
     "consumables":    "Sirvi ja osta turgutusi (korduvostetavad boostid)",
     "vanity":         "Staatusepood - osta ja kanna kosmeetilisi tiitleid",
+    "lootbox":        "Ava õnnekast - juhuslik auhind müntide või boonuse näol",
 }
 
 # ---------------------------------------------------------------------------
@@ -153,6 +154,9 @@ HELP_CATEGORIES: dict[str, dict] = {
             ("/leaderboard", "TipiBOTi edetabel - kes on kõige rikkam?"),
             ("/shop", "Sirvi TipiBOTi poodi"),
             ("/buy <item>", "Osta ese TipiBOTi poodist"),
+            ("/consumables", "Osta korduvostetavaid turgutusi (ajutised boostid)."),
+            ("/lootbox", "Ava õnnekast (1000 ⬡) - juhuslik auhind: mündid või ajutine boonus."),
+            ("/vanity", "Staatusepood - osta ja kanna kosmeetilisi tiitleid (näha /profile-l)."),
             ("/request <amount> <reason> [target]", "Saada crowdfundingu taotlus. Keegi saab 'Toeta' nuppu vajutades raha kanda (taotlus kehtib 5 minutit)."),
             ("/reminders", "DM meeldetuletused on vaikimisi sees. Kasuta seda käsku, et lülitada sisse/välja, milliseid käsklusi meelde tuletada."),
         ],