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be spent, gambled or given until withdrawn - the deliberate trade-off against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

README.md

@@ -336,9 +336,12 @@ The **ECONOMY** role is granted on your first EXP award (i.e. first successful e
 | `/rank [@user]` | EXP total, current level, progress bar to next level, leaderboard rank. |
 | `/stats [@user]` | Lifetime statistics: economy totals, work/beg counts, gambling records, crime/heist history, social totals, best streak. |
 | `/cooldowns` | All cooldowns at a glance with live Discord timestamps. Shows jail timer if jailed. |
-| `/leaderboard` | Paginated leaderboard with 6 tabs: 🪙 Coins, 📊 EXP, 🏆 Season EXP, 🔥 Prestige, 🎲 Wagered, 🎣 Fish caught. House pinned at #0 on coins tab. |
+| `/leaderboard` | Paginated leaderboard with 6 tabs: 🪙 Coins (net worth = wallet + bank), 📊 EXP, 🏆 Season EXP, 🔥 Prestige, 🎲 Wagered, 🎣 Fish caught. House pinned at #0 on coins tab. |
 | `/shop` | Browse all items by tier. Shows owned status, Anticheat charges remaining, and level lock for T2/T3. |
 | `/buy <item>` | Purchase an item by name (partial match accepted). |
+| `/bank` | View your vault. Banked coins are **rob-proof** (`/rob` and `/heist` only touch liquid balance) but earn no Bot Farm interest and can't be spent until withdrawn. |
+| `/deposit <amount>` | Move coins from your wallet into the bank. `all` deposits everything liquid. |
+| `/withdraw <amount>` | Move coins from the bank back to your wallet. `all` withdraws everything banked. |
 | `/consumables` | Buy repeatable, expiring boosts (earn ×2, EXP ×2, or an instant cooldown wipe). A recurring coin sink. |
 | `/lootbox` | Open a mystery box for **1 000 ⬡**. Weighted random reward: coin tiers (usually a small net loss), a 30-min earn/EXP ×2 buff, or a rare jackpot. |
 | `/vanity` | Status shop — buy and equip cosmetic badges/titles (shown on `/profile`). Coins are **burned**, not recirculated. No gameplay effect. |