Feat/backend implement vendor contract management module #295

[OlufunbiIK]

Title: Add Vendor Contracts Module with CRUD and Expiry Notifications

Description:
This PR introduces a new vendor-contracts module to manage long-term contracts with vendors who supply assets or maintenance services.

Key Features Implemented

Database Layer

Created vendor_contracts table with fields: supplierId, contractName, startDate, endDate, fileUrl, isActive, timestamps.

Added indexes on supplierId and endDate for efficient lookups.

Entity & DTOs

Defined VendorContract entity with validation rules.

Created DTOs for Create, Update, and Query operations.

CRUD Operations

Implemented service methods for create, read, update, and delete contracts.

Built REST controller with endpoints for managing vendor contracts.

Added pagination and search capabilities.

Expiry Management

Added method to retrieve contracts expiring within a configurable number of days.

Implemented scheduled task to check for expiring contracts daily.

Integrated notification service for near-expiry alerts (currently logs to console; pluggable for email/other channels).

Next Steps

Integrate with a real notification service (e.g., email, Slack, or push).

Add role-based access control to secure contract management.

Extend with supplier relationship (FK to suppliers table) once supplier module is available.

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