feat: add optimistic locking and improve type safety

- Implement optimistic locking for deposit balance updates
  - Prevent race conditions in concurrent deposit requests
  - Add automatic retry with exponential backoff (max 3 attempts)
  - Add version column to user_deposits table

- Improve type safety across codebase
  - Add explicit types for Namecheap API responses
  - Add typed function arguments (ManageDepositArgs, etc.)
  - Remove `any` types from deposit-agent and tool files

- Add reconciliation job for balance integrity verification
  - Compare user_deposits.balance vs SUM(confirmed transactions)
  - Alert admin on discrepancy detection

- Set up test environment with Vitest + Miniflare
  - Add 50+ test cases for deposit system
  - Add helper functions for test data creation

- Update documentation
  - Add migration guide for version columns
  - Document optimistic locking patterns

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

migrations/002_add_version_columns.sql

@@ -0,0 +1,28 @@
+-- Migration 002: Add version column for Optimistic Locking
+-- Purpose: Enable concurrent transaction safety for deposit balance updates
+-- Date: 2026-01-19
+-- Reference: CLAUDE.md "Transaction Isolation & Optimistic Locking"
+
+-- Problem:
+--   D1 batch() is not a true transaction - partial failures can cause data inconsistencies
+--   in financial operations (balance updates + transaction records)
+
+-- Solution:
+--   Optimistic Locking pattern with version column
+--   - Version is incremented on every balance UPDATE
+--   - UPDATE checks current version to detect concurrent modifications
+--   - Automatic retry with exponential backoff (max 3 attempts)
+
+-- Add version column to user_deposits
+ALTER TABLE user_deposits ADD COLUMN version INTEGER NOT NULL DEFAULT 1;
+
+-- Create index for efficient version checking
+CREATE INDEX IF NOT EXISTS idx_deposits_user_version ON user_deposits(user_id, version);
+
+-- Verification:
+--   SELECT user_id, balance, version FROM user_deposits LIMIT 5;
+
+-- Rollback instructions (manual execution required):
+--   DROP INDEX IF EXISTS idx_deposits_user_version;
+--   -- Note: SQLite doesn't support DROP COLUMN directly in older versions
+--   -- If needed, recreate table without version column and copy data