fix(catalog): enforce category tree and nullable-field update invariants

[khaira777]

Summary

Category writes accept empty/whitespace names, missing parents, self-parenting, and cycles. Deletion does not define child handling and can reassign products to the category being deleted. COALESCE-based updates also prevent explicit clearing of nullable catalog fields.

Current behavior

• main/routes/categories.ts:59-107,135-148 accepts invalid parent relationships and empty names.
• Category deletion does not reject reassign_to equal to the category being deleted.
• COALESCE update expressions in category/product/add-on routes treat explicit null as “preserve existing value”.

Expected behavior

The category hierarchy should remain valid, deletion/reassignment behavior should be explicit and transactional, and omitted fields should be distinguishable from explicit null-clears.

Acceptance criteria

• Trim and require non-empty category names.
• Require existing active parents and reject self/ancestor cycles.
• Reject reassignment to the category being deleted.
• Define and test child-category handling on parent deletion.
• Use presence-aware update semantics so explicit null can clear nullable catalog fields.

Relevant files

main/routes/categories.ts, main/routes/products.ts, main/routes/addon-groups.ts