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/store.py

@@ -106,6 +106,7 @@ class UserData(TypedDict, total=False):
     last_streak_date: str | None   # ISO date "YYYY-MM-DD"
     items: list[str]
     item_uses: dict                 # {item_id: remaining_uses} for consumables
+    active_buffs: dict              # {buff_kind: expiry_iso} for timed consumable boosts
     jailed_until: str | None       # ISO datetime or None
     jailbreak_used: bool
     reminders: list[str]           # command names user wants DM reminders for
@@ -159,6 +160,7 @@ def _default_user() -> UserData:
         "last_streak_date": None,
         "items": [],
         "item_uses": {},
+        "active_buffs": {},
         "jailed_until": None,
         "jailbreak_used": False,
         "reminders": ["daily", "work", "beg", "crime", "rob"],