[imorland]

Fixes #5236

Changes proposed in this pull request:
Stop offering reordering within the alphabetical secondary-tag list while keeping moves between primary and secondary tags. Editing a tag now preserves its kind; changing isPrimary alone returns 422, while an unchanged value remains accepted. An upgrade migration repairs incorrect flags and duplicate positions.

Reviewers should focus on:

• Repairing flags from positions and renumbering each tag list.
• The API rule for changing a tag’s kind.

Screenshot
Before: dragging Charlie temporarily changes the secondary list, then the order is discarded.

After: the secondary list stays alphabetical during the same drag.

Before recording · After recording

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Frontend changes: tests are green (run yarn test in js/).
• Frontend changes: tests have been added, or are not appropriate here.
• Backend changes: tests are green (run composer test).
• Backend changes: tests have been added, or are not appropriate here.
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite).
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.

Required changes:

• Related documentation PR: docs: clarify tag ordering and kind updates docs#708