TASK: Separate categories and topic categories from tags

[johnhenley]

Categories and topic categories are currently handled as tags with an 'isCategory' flag.
Separate them out to be first-class entites.

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Previously discussed in #1429

^{Originally posted by johnhenley March 18, 2024}
Topics can have tags and/or categories. Categories are currently stored as tags within the module bit with "is category" flag. This is confusing and poor design. Either promote categories to first-class entity or remove this feature. Do you use categories???