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/utils/optimistic-lock.ts

@@ -32,6 +32,13 @@ import { createLogger } from './logger';
 
 const logger = createLogger('optimistic-lock');
 
+/**
+ * Type guard for Error with captureStackTrace method
+ */
+interface ErrorWithCapture {
+  captureStackTrace?: (target: object, constructor?: Function) => void;
+}
+
 /**
  * Custom error for optimistic lock failures
  */
@@ -40,10 +47,9 @@ export class OptimisticLockError extends Error {
     super(message);
     this.name = 'OptimisticLockError';
     // Maintain proper stack trace for debugging (Node.js only, not available in Workers)
-    // eslint-disable-next-line @typescript-eslint/no-explicit-any
-    if (typeof (Error as any).captureStackTrace === 'function') {
-      // eslint-disable-next-line @typescript-eslint/no-explicit-any
-      (Error as any).captureStackTrace(this, OptimisticLockError);
+    const ErrorConstructor = Error as unknown as ErrorWithCapture;
+    if (typeof ErrorConstructor.captureStackTrace === 'function') {
+      ErrorConstructor.captureStackTrace(this, OptimisticLockError);
     }
   }
 }