feat(wallet-service): add migration for memo column type change to TEXT

Change wallet_ledger_entries.memo from VARCHAR(500) to TEXT to support longer settlement memos.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/services/wallet-service/prisma/migrations/20260107000000_memo_to_text/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable: Change memo column from VARCHAR(500) to TEXT
+ALTER TABLE "wallet_ledger_entries" ALTER COLUMN "memo" TYPE TEXT;