fix(maintenance): prevent backup source and destination recursion, add dry-run and cleanup trap

[ashishsinghbora]

Summary

• Prevented recursive backup bloat when output destination is inside or equal to the source directory (auto-calculates relative path and passes tar --exclude flags).
• Added -n / --dry-run option to preview planned backup paths and exclusions without creating files.
• Added EXIT/INT/TERM cleanup trap that automatically deletes corrupted or incomplete archives if tar fails or is interrupted.
• Validates source path existence and directory type prior to execution, rejecting root / backups without explicit targeting.
• Safely handles spaces, nested directories, and Unicode characters in file paths.

Verification

• Tested backup of directory containing spaces, subdirectories, and Unicode characters.
• Verified nested output directory (backups/) is excluded from the generated archive.
• Verified dry-run does not write to the filesystem.
• Verified nonexistent source directories exit non-zero.
• Verified ShellCheck passes without warnings.
• Verified test suite passes.

[ashishsinghbora]

Code review: verified exclusion logic stops destination directory recursion, tested spaces/Unicode handling, and verified cleanup traps on failure.