refactor: apply new utilities and constants across codebase

P0 fixes:
- KV Cache migration: security.ts now delegates to kv-cache.ts (74% code reduction)
- Environment validation: index.ts validates env on first request
- Type safety: optimistic-lock.ts removes `as any` with proper interface

P1 improvements:
- Constants applied to deposit-agent.ts (TRANSACTION_STATUS, TRANSACTION_TYPE)
- Constants applied to callback-handler.ts (CALLBACK_PREFIXES)
- Constants applied to domain-tool.ts (MESSAGE_MARKERS)

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

src/tools/domain-tool.ts

@@ -9,6 +9,7 @@ import { retryWithBackoff, RetryError } from '../utils/retry';
 import { createLogger, maskUserId } from '../utils/logger';
 import { getOpenAIUrl } from '../utils/api-urls';
 import { ERROR_MESSAGES } from '../constants/messages';
+import { MESSAGE_MARKERS } from '../constants';
 
 const logger = createLogger('domain-tool');
 
@@ -796,7 +797,7 @@ async function executeDomainAction(
           domain: domain,
           price: price
         });
-        return `__KEYBOARD__${keyboardData}__END__
+        return `${MESSAGE_MARKERS.KEYBOARD}${keyboardData}${MESSAGE_MARKERS.KEYBOARD_END}
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