Add custom attachments directory naming and sequential filename indentation features

[Copilot]

This PR implements two highly requested features that enhance file organization and naming flexibility:

🗂️ Feature 1: Custom Attachments Directory Naming

Adds support for customizing the attachments directory name using post properties, similar to how post_dirname_format works for post directories.

New Configuration Option:

KTOOLBOX_JOB__POST_STRUCTURE__ATTACHMENTS_DIRNAME_FORMAT={title}

Example Usage:

# Use post title as attachments folder name
KTOOLBOX_JOB__POST_STRUCTURE__ATTACHMENTS_DIRNAME_FORMAT={title}

# Use post ID and title combined  
KTOOLBOX_JOB__POST_STRUCTURE__ATTACHMENTS_DIRNAME_FORMAT={id}_{title}

# Use published date and title
KTOOLBOX_JOB__POST_STRUCTURE__ATTACHMENTS_DIRNAME_FORMAT={published}_{title}

Result:

Post_Directory/
├─ content.txt
├─ post.json
└─ Title_of_Post/         # Instead of "attachments"
   ├─ 1.jpg
   └─ 2.png

🔢 Feature 2: Sequential Filename Indentation

Adds the ability to preserve original filenames while using sequential numbering, addressing the limitation where sequential_filename=True completely replaced original names.

New Configuration Option:

KTOOLBOX_JOB__SEQUENTIAL_FILENAME_INDENTATION=True

Comparison:

• Without indentation: 1.png, 2.jpg, 3.gif
• With indentation: 1_OriginalImage.png, 2_PhotoFile.jpg, 3_Animation.gif

🔧 Implementation Details

• Backward Compatible: All existing functionality preserved, new features are opt-in
• Integrates Seamlessly: Works with existing features like sequential_filename_excludes, filename_format, etc.
• Minimal Changes: Surgical modifications following the codebase patterns
• Comprehensive Testing: 15 new test cases covering various scenarios and edge cases
• Full Documentation: Updated FAQ, configuration guide, and auto-generated reference docs

🧪 Testing

All tests pass including:

• 8 test cases for custom attachments directory naming
• 7 test cases for sequential filename indentation
• All existing functionality regression tests
• Live demo showing features working individually and together

📖 Documentation

• Added FAQ entries with detailed examples and use cases
• Updated configuration guide with practical examples
• Enhanced docstrings for auto-generated reference documentation

Both features address real user needs for better file organization and naming flexibility, while maintaining the simplicity and reliability that KToolBox users expect.

Fixes #350.

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