Add --modified-within flag to limit files by last modified date in backups

[Copilot]

This PR adds a new optional --modified-within=AGE flag to the backup command that optimizes backup performance by limiting file consideration to those modified within a specified timeframe.

Problem

The immutable-backups.sh script currently compares all files from the source directory against every previous backup, resulting in very long runtimes as the number of previous backups increases.

Solution

The new --modified-within=AGE flag:

• Works for both full and incremental backups
• Passes the age value as --max-age=AGE to rclone, using rclone's standard age syntax (e.g., 30d, 72h, 12w)
• Maintains backward compatibility - when omitted, behavior is unchanged
• Is compatible with rclone v1.55.1 and later

Examples

# Take an incremental backup considering only files modified in the last 30 days
./immutable-backups.sh backup --local=/path/to/files --remote=your-remote: --incremental --modified-within=30d --dry-run

# Take a full backup considering only files modified in the last 72 hours
./immutable-backups.sh backup --local=/path/to/files --remote=your-remote: --modified-within=72h --dry-run

Changes Made

• Added argument parsing for --modified-within=AGE flag
• Updated both incremental and full backup rclone commands to include --max-age when specified
• Enhanced help text with usage documentation and examples
• Maintained full backward compatibility

Fixes #3.

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