Add Django Signals support for backup and restore operations

[Copilot]

This PR implements comprehensive Django Signals support for django-dbbackup, allowing users to hook into backup and restore operations to perform custom actions.

New Signals

Database Operations

• dbbackup.signals.pre_backup - Sent before a database backup starts
• dbbackup.signals.post_backup - Sent after a database backup completes
• dbbackup.signals.pre_restore - Sent before a database restore starts
• dbbackup.signals.post_restore - Sent after a database restore completes

Media Operations

• dbbackup.signals.pre_media_backup - Sent before a media backup starts
• dbbackup.signals.post_media_backup - Sent after a media backup completes
• dbbackup.signals.pre_media_restore - Sent before a media restore starts
• dbbackup.signals.post_media_restore - Sent after a media restore completes

Usage Example

from django.dispatch import receiver
from dbbackup import signals

@receiver(signals.pre_backup)
def backup_started(sender, database, **kwargs):
    print(f"Starting backup of database: {database['NAME']}")

@receiver(signals.post_backup)  
def backup_completed(sender, database, filename, **kwargs):
    print(f"Completed backup of {database['NAME']} to {filename}")

Signal Arguments

Each signal provides relevant context:

• Database signals: database config, connector, servername, filename, storage
• Media signals: servername, filename, storage

Common use cases include:

• Sending notifications when backups complete
• Logging backup/restore operations
• Integration with monitoring systems
• Custom validation before restores
• Triggering webhooks to external services

Implementation Details

• All signals are defined in dbbackup.signals module
• Signals are sent at appropriate points in backup/restore command methods
• Full backward compatibility maintained
• Comprehensive test coverage (9 new tests)
• Detailed documentation with examples

The implementation uses minimal, surgical changes to existing command classes while providing powerful extensibility for users who need to customize backup/restore workflows.

Fixes #613.

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Pull Request Overview

This PR implements Django Signals support for the django-dbbackup package, allowing users to hook into backup and restore operations for both database and media files. The implementation adds 8 new signals that are sent before and after each operation type.

• Adds comprehensive Django signals for all backup and restore operations
• Provides consistent signal arguments with relevant context data
• Maintains full backward compatibility with existing functionality

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
dbbackup/signals.py	Defines all 8 new Django signals for backup/restore operations
dbbackup/management/commands/dbbackup.py	Integrates pre_backup and post_backup signals
dbbackup/management/commands/dbrestore.py	Integrates pre_restore and post_restore signals
dbbackup/management/commands/mediabackup.py	Integrates pre_media_backup and post_media_backup signals
dbbackup/management/commands/mediarestore.py	Integrates pre_media_restore and post_media_restore signals
tests/test_signals.py	Comprehensive test suite covering all signal functionality
docs/src/signals.md	Complete documentation with usage examples and API reference
docs/mkdocs.yml	Adds signals documentation to navigation
CHANGELOG.md	Documents the new signals feature