Records the decisions that are not visible from the code alone:
- why section_field has no parent_id, and what reuse that buys;
- that template_field.sort is the single source of display order, and
that repeats and ties are deliberately legal;
- that templates reference library fields rather than copying them, which
is what makes a rename propagate;
- that TiDB parses FOREIGN KEY and ignores it, so the integrity lives in
the application layer.
Also adds the API table, the frontend route table, and the seed script.
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