[ADD] Implement Discord Notification System for outstanding payments and add tax message field for owners

[Geuthur]

Description

Added

• Discord Notification System, Automated alerts for outstanding payments at both the Alliance and Corporation levels.
• tax_message field for Owner to specify custom First Message.

Configuration
The frequency of these alerts can be managed through the following setting:

TAXSYSTEM_NOTIFICATION_EXPIRATION_DAYS = "1"

Defines the timeframe (in days) before a notification expires. The system will only trigger a new Discord alert for the outstanding balance once this period has elapsed. This prevents notification fatigue while ensuring consistent follow-up on unpaid dues.

Type of Changes

Please select the type of changes made in this pull request:

• Bug Fix (non-breaking change which fixes an Issue)
• New Feature (non-breaking change)
• Other (please specify):

Checklist

Please ensure the following before submitting your pull request:

• I have read the contributing guidelines.
• My code follows the code style of this project.
• I have performed a self-review of my code.
• I have commented my code where necessary.
• All new and existing tests passed.

[codecov]

Codecov Report

❌ Patch coverage is 91.79104% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
taxsystem/tasks.py	84.21%	2 Missing and 7 partials ⚠️
taxsystem/models/base.py	80.00%	1 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
taxsystem/app_settings.py	100.00% <100.00%> (ø)
taxsystem/helpers/discord.py	100.00% <100.00%> (ø)
taxsystem/models/__init__.py	100.00% <100.00%> (ø)
taxsystem/models/alliance.py	79.23% <100.00%> (+0.16%)	⬆️
taxsystem/models/corporation.py	85.64% <100.00%> (+0.07%)	⬆️
taxsystem/models/base.py	84.92% <80.00%> (+0.79%)	⬆️
taxsystem/tasks.py	82.06% <84.21%> (+0.66%)	⬆️

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