Add bulk import functionality for tags with duplicate handling

[ngAlexander9]

Description

• Added bulk_import_tags endpoint (/v1/tags/bulk) that allows clients to import multiple tag groups and tags in one request, with clear summary reporting of created groups, created tags, skipped tags (due to duplicates), and warnings. Avoids having to add each tag individually (400+ queries!).
• Updated the add_tag_to_group endpoint to accept a skip_duplicates query parameter. When set to true, duplicate tags are skipped instead of causing an error.

Motivation

Supports slaclab/react-squirrel#84

Screenshots

Pre-merge checklist

• Code works interactively
• Code contains descriptive docstrings
• New/changed functions and methods are covered in the test suite where possible
• Test suite passes locally
• Test suite passes on GitHub Actions