Fix critical race conditions and performance issues

- Implement atomic check-and-create pattern in ensureEmailJob with exponential backoff
- Fix import mismatch by exporting processJobById from index.ts
- Enable database indexes for status+scheduledAt and priority+createdAt fields
- Standardize string conversion for consistent ID handling throughout codebase
- Fix TypeScript compilation errors in collection indexes and variable scope

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/jobs/processEmailJob.ts

@@ -9,6 +9,10 @@ export interface ProcessEmailJobInput {
    * The ID of the email to process
    */
   emailId: string | number
+  /**
+   * Optional unique constraint helper to prevent duplicate jobs
+   */
+  uniqueKey?: string
 }
 
 /**