feat: add parent tag support for hierarchical tag structures

[zagah]

Summary

• Adds optional parent field to Tag, TagCreateRequest, and TagUpdateRequest models
• Exposes parent as an optional parameter on paperless_tags_create and paperless_tags_update MCP tools
• No breaking changes — parent defaults to null everywhere

Background

Paperless-ngx supports nested/hierarchical tags via a parent ID on the API. This PR makes that capability accessible through the MCP interface, allowing AI assistants to create and manage tag hierarchies (e.g. a Taxes parent tag with 2023, 2024 children).

Test plan

• Build succeeds with zero warnings
• Existing tag tests pass unchanged
• Manual: create a child tag with parent=<id> and verify Paperless stores the hierarchy correctly

🤖 Generated with Claude Code

[barryw]

Code Review — Parent Tag Support

Overview

Clean implementation adding parent parameter to tag Create/Update tools and the Tag model for hierarchical tag structures. Code correctness is good — WhenWritingNull is set globally so null parents won't accidentally unset values on PATCH.

Issues

#	Severity	Issue
1	Blocker	No tests for new behavior. Zero tests cover Create with parent or Update with parent. Project convention requires test coverage for new features. At minimum, add tests for: Create_WithParent_ReturnsCreatedTagWithParent and Update_WithParent_SendsParentId.
2	Low	No way to explicitly unset a parent (move tag back to root) due to WhenWritingNull. Should be documented in the parent parameter [Description] on Update, or tracked as a follow-up issue.
3	Nit	TestFixtures.Tags.CreateTag doesn't accept an optional parent parameter, so fixtures can't exercise hierarchy deserialization.

What's Good

• Tool naming follows underscore convention ✓
• No breaking changes, no security concerns, no performance impact
• Feature is well-scoped and the model/tool changes are minimal

Recommendation

Add tests for the new parent parameter on create and update before merge.