feat: Improve handling of deep taxonomies (perf + limits + bugfixes)

[bradenmacdonald]

This is a fix for openedx/modular-learning#257 .

In short: the API was very inconsistent with how it handles deeply-nested tags, and arguably the behavior was buggy.

Approach: This PR updates the Tag data model to store depth and lineage as columns, rather than computing them dynamically. Then, I rewrote all the queries to support unlimited tag depth. With the depth and lineage columns available, we can perform all the same queries very efficiently without having to hard-code things like parent__parent__parent__... that assume a certain depth limit. Now all the API methods work with an unlimited tag depth.

Actual depth limit: This PR also clarifies the definition of TAXONOMY_MAX_DEPTH and actually enforces it to limit the allowed depth to six levels. (Before this, no limit was enforced when creating tags. A limit of 3 levels was enforced when reading tags from multiple levels at once, but it didn't work well below the root.)

Perfomance: pretty much on par with the main branch in every way I could measure. Significantly faster than my CTE approach #510 .

AI disclosure: Claude assisted with this PR.

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