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be spent, gambled or given until withdrawn - the deliberate trade-off against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/leaderboards.py

@@ -7,11 +7,17 @@ from . import house
 from .levels import get_level
 
 
+def _net_worth(r: dict) -> int:
+    """Coins that count toward wealth: liquid balance + banked vault."""
+    return (r.get("balance", 0) or 0) + (r.get("bank_balance", 0) or 0)
+
+
 async def get_leaderboard(top_n: int | None = 10) -> list[tuple[str, int]]:
-    """Return top_n (user_id_str, balance) pairs sorted descending."""
+    """Return top_n (user_id_str, net_worth) pairs sorted descending.
+    Net worth = balance + bank_balance, so banking coins does not hide them."""
     records = await pb_client.list_all_records()
     result = sorted(
-        ((r["user_id"], r.get("balance", 0)) for r in records if r.get("user_id")),
+        ((r["user_id"], _net_worth(r)) for r in records if r.get("user_id")),
         key=lambda x: x[1],
         reverse=True,
     )
@@ -97,8 +103,8 @@ async def get_all_leaderboards() -> dict[str, list[tuple]]:
         return sorted(users, key=keyfn, reverse=True)
 
     return {
-        "coins":    [(r["user_id"], r.get("balance", 0))
-                     for r in desc(lambda r: r.get("balance", 0))],
+        "coins":    [(r["user_id"], _net_worth(r))
+                     for r in desc(_net_worth)],
         "exp":      [(r["user_id"], r.get("exp", 0), get_level(r.get("exp", 0)))
                      for r in desc(lambda r: r.get("exp", 0))],
         "season":   [(r["user_id"], r.get("season_total_exp", 0), r.get("prestige_level", 0))
@@ -125,10 +131,11 @@ async def get_economy_stats() -> dict[str, int]:
         uid = r.get("user_id")
         if not uid:
             continue
-        bal = r.get("balance", 0) or 0
-        total += bal
+        # Banked coins are still part of the money supply.
+        worth = (r.get("balance", 0) or 0) + (r.get("bank_balance", 0) or 0)
+        total += worth
         if uid == house_id:
-            house_balance = bal
+            house_balance = worth
         else:
             player_count += 1
     return {