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/index.ts

@@ -10,13 +10,49 @@ import { handleProvisionQueue, handleProvisionDLQ } from './server-provision';
 import { timingSafeEqual } from './security';
 import { createLogger } from './utils/logger';
 import { notifyAdmin } from './services/notification';
+import { validateEnv } from './utils/env-validation';
 import { Hono } from 'hono';
 
 const logger = createLogger('worker');
 
+// Environment validation flag (checked once per instance)
+let envValidated = false;
+
 // Hono app with Env type
 const app = new Hono<{ Bindings: Env }>();
 
+// Environment validation middleware (runs once per worker instance)
+app.use('*', async (c, next) => {
+  if (!envValidated) {
+    // Cast to Record<string, unknown> for validation
+    const result = validateEnv(c.env as unknown as Record<string, unknown>);
+    if (!result.success) {
+      logger.error('Environment validation failed on startup', new Error('Invalid configuration'), {
+        errors: result.errors,
+      });
+      return c.json({
+        error: 'Configuration error',
+        message: 'The worker is not properly configured. Please check environment variables.',
+        details: result.errors,
+      }, 500);
+    }
+
+    // Log warnings but continue
+    if (result.warnings.length > 0) {
+      logger.warn('Environment configuration warnings', { warnings: result.warnings });
+    }
+
+    logger.info('Environment validation passed', {
+      environment: c.env.ENVIRONMENT || 'production',
+      warnings: result.warnings.length,
+    });
+
+    envValidated = true;
+  }
+
+  return await next();
+});
+
 // Health check (public - minimal info only)
 app.get('/health', () => handleHealthCheck());