feat(economy): add /consumables shop of repeatable, expiring boosts

The economy had many coin faucets but almost no sinks: the /shop is
one-time ownership, and gambling/rob fines route to the house (which
players drain back via jackpots and heists), so they recirculate rather
than destroy coins. Result: steady inflation.

Add a consumables shop as a true recurring sink - buying destroys the
coins and grants a temporary boost, so there's always something to spend
on after gear is maxed:

  - Energiajook XL (500) - 1h of 2x earnings on /work, /beg, /crime
  - XP jook (500)        - 1h of 2x EXP
  - Kohv (300)           - instantly clears all cooldowns

Timed buffs live in a new active_buffs field ({kind: expiry_iso}), pruned
on read; rebuying extends the timer. Effects hook where they belong:
earn_mult in income.do_work/do_beg/do_crime, exp_buff_mult in
levels.award_exp; kohv is self-contained. New /consumables command browses
the menu (with active buffs) or buys a boost. Covered by 9 tests.

Note: active_buffs is a new PocketBase field - run
scripts/sync_pb_schema.py before deploying or buffs won't persist.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

core/economy/levels.py

@@ -5,6 +5,7 @@ from __future__ import annotations
 import math
 
 from .store import _locked_by, _prestige_mult, get_user, _commit
+from .consumables import exp_buff_mult
 
 
 # ---------------------------------------------------------------------------
@@ -69,7 +70,7 @@ async def award_exp(user_id: int, amount: int) -> dict:
     """Add EXP to a user. Applies prestige exp_mult. Returns old_level, new_level, total exp."""
     user = await get_user(user_id)
     _, exp_mult = _prestige_mult(user)
-    gained = max(1, int(amount * exp_mult))
+    gained = max(1, int(amount * exp_mult * exp_buff_mult(user)))
     old_exp = user.get("exp", 0)
     new_exp = old_exp + gained
     old_level = get_level(old_exp)