Let a subsection hang under another subsection

[ssavutu]

A&E asked for a Food subsection holding Beer Reviews, Wine Reviews, Restaurant Reviews and Cooking. That is a third level the taxonomy did not have: parent_slug had to name a section, and the rollup was a single non-recursive hop.

The shape

Depth is a property of the parent chain, not a new kind. Both levels stay kind='subsection', so everything that switches on kind — the public site's slug router, the article filters, the sections screen — keeps working unchanged. MaxTaxonomyDepth caps the chain at three, because the walks are recursive over rows editors can edit.

What actually mattered

The rollup is now transitive. This is the load-bearing change. One hop would have counted Beer Reviews for Food while dropping it from A&E, leaving a section quietly shorter than the sum of its subsections — the same silent-omission failure the alias seeding exists to fix. Recounts walk every ancestor for the same reason, and a subsection resolves to its root section rather than its parent, since that is what ?section_slug= is checked against.

A subsection can be a container. /food matches its descendants; matching food alone would render it empty, as its articles are filed under the four categories beneath it. /v1/subsections/<slug>/articles also returns subsections now, so a middle row gets the link strip a section page has.

The guards widened. Delete and slug-change only asked about sections. Deleting Food would have stranded four rows pointing at a parent that no longer exists.

One behaviour change worth reviewing: converting a section that has children into a subsection used to be refused outright, on the premise that a subsection could not parent one. It is now allowed when the resulting subtree still fits in three levels. The existing test asserting the old rule was updated to assert the depth rule instead.

Migration

SeedFoodSubsection creates Food and moves the four rows, once, under its own flag — legacy_subsections_seeded is already set in production. The move is guarded on the current parent, so a row an editor pulls back out stays where they put it.

Cooking is the one visible change to a section page readers use today: unlike the three review categories it is a live, visible subsection, so moving it takes its link out of A&E's strip and puts it in Food's. That is the intent.

Testing

Tree-walk unit tests including cycle termination, plus DB-backed tests for the three-level rollup, the fourth-level rejection, and the seed's idempotence against an editor's change. Full Go suite green against MariaDB; frontend 19/19.

Needs DrexelTriangle/Scalene#94 for the public side, which links a subsection's own children in the nav strip.

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