feat(economy): add /bank vault (rob-proof storage) + net-worth leaderboard

A strategic counterpart to /rob: coins moved to the bank are safe from /rob and
/heist (which only touch liquid balance) but earn no Bot Farm interest and can't
be spent, gambled or given until withdrawn - the deliberate trade-off against
keeping coins liquid.

- New bank.py (do_deposit/do_withdraw), bank_balance schema field.
- /bank (view), /deposit, /withdraw commands ('all' supported), with db_error
  handling; /balance shows the vault when non-zero.
- Coins leaderboard and /status money-supply now count net worth
  (balance + bank_balance), so banking never hides you from the board or the
  supply metric.
- Season reset wipes bank_balance too (no cross-season wealth hiding).

10 tests: deposit/withdraw math, guards, coin conservation, rob cannot touch the
vault, and net-worth accounting on the leaderboard + stats.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013VbAVsrZuYesea99mPMmPT

core/economy/store.py

@@ -114,7 +114,8 @@ HEIST_JAIL     = timedelta(hours=1, minutes=30)
 # User schema
 # ---------------------------------------------------------------------------
 class UserData(TypedDict, total=False):
-    balance: int
+    balance: int                    # liquid coins - spendable, gamblable, robbable
+    bank_balance: int               # vaulted coins - safe from /rob and /heist, not spendable until withdrawn
     exp: int                        # lifetime EXP (resets each season)
     last_daily: str | None
     last_work: str | None
@@ -177,6 +178,7 @@ class UserData(TypedDict, total=False):
 def _default_user() -> UserData:
     return {
         "balance": 0,
+        "bank_balance": 0,
         "exp": 0,
         "last_daily": None,
         "last_work": None,