Add wallet_type column to distinguish exchanges from wallets

Exchanges (can receive bank deposits) vs wallets (cold storage only).
Supports ?type= filter on GET /api/wallets.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

resources/migrations/20260308100000-add-wallet-type.down.sql

@@ -0,0 +1 @@
+ALTER TABLE wallets DROP COLUMN wallet_type;

resources/migrations/20260308100000-add-wallet-type.up.sql

@@ -0,0 +1,4 @@
+ALTER TABLE wallets ADD COLUMN wallet_type TEXT NOT NULL DEFAULT 'exchange';
+--;;
+UPDATE wallets SET wallet_type = 'wallet'
+WHERE name IN ('Cold', 'Coldcard', 'Nunchuk Multi Sig');