feat: Implement payroll scheduler backend wiring

[meshackyaro]

Description

This PR implements the backend infrastructure for automated payroll scheduling, connecting the frontend PayrollScheduler components to a persistent database and cron-based execution system.

Changes

Database Layer

• Added schedules table migration with frequency, time, payment config, and execution tracking
• Added execution_history table migration for audit logging
• Created verification scripts for schema validation

Service Layer

• ScheduleService: CRUD operations for schedules with next run calculation

  • calculateNextRun(): Handles once, weekly, biweekly, monthly frequencies
  • createSchedule(): Validates and persists schedule configurations
  • getActiveSchedules(): Retrieves schedules with pagination
  • cancelSchedule(): Cancels pending schedules with authorization checks
  • updateAfterExecution(): Updates schedule state after execution

• ScheduleExecutor: Cron-based automated payment execution

  • processDueSchedules(): Queries and processes due schedules every minute
  • executeSchedule(): Builds and submits Stellar bulk payment transactions
  • recordExecution(): Logs execution results to execution_history
  • initialize(): Sets up cron job with graceful shutdown

API Layer

• POST /api/schedules: Create new payroll schedule
• GET /api/schedules: List active schedules with pagination
• DELETE /api/schedules/:id: Cancel pending schedule
• Zod validation schemas for request validation
• Comprehensive error handling with structured error responses

Integration

• Wired ScheduleExecutor into server startup
• Added graceful shutdown handling for cron job
• Integrated with existing authentication and authorization middleware
• Connected to Stellar blockchain via StellarService

Error Handling

• Request validation with Zod schemas
• Database transaction safety with rollback
• Stellar error parsing for meaningful error messages
• Isolated execution (one failure doesn't block others)

Testing

• Unit tests for all service methods
• Manual test scripts for verification
• Comprehensive validation test coverage

Documentation

• Migration verification guide
• Schedule executor implementation guide
• Database setup instructions

Closes #82

[Wilfred007]

Hi @meshackyaro kindly resolve conflicts

[meshackyaro]

conflicts resolved please check