add_child_collection: enforce DAG by rejecting indirect cycles

Problem

A Collection nests other Collections (CollectionMember parent→child) and is described as a DAG (acyclic). add_child_collection only guards the direct self-loop:

• parent_id == child_id → rejected
• already-a-child → ConflictError

There is no check for indirect cycles. Nesting B under A, C under B, then A under C closes a 3-hop loop, and the code accepts it. So acyclicity is enforced only at depth 1.

Impact

Any recursive walk of the nesting then loops (infinite recursion / stack blow-up). Live consumers:

• children expansion in the collection response (collection_service ~L493)
• get_effective_access_level, which walks a collection's relations

Footgun in normal use; a DoS vector if collection nesting is ever exposed to less-trusted writers.

Fix

In add_child_collection, reject the edge if parent is already a descendant of child (ancestor/descendant check), not just the direct parent_id == child_id case.

Scope note

Missing validation, not a data-model flaw — surfaced while pressure-testing the context/run/curation model. Independent of that ADR.

Migrated from the internal tracker, where it was #17, opened 2026-06-25.

Start by locating add_child_collection and the children expansion in collection_service around L493. Trace how parent and child relations are traversed, then verify that adding an edge from a collection to one of its descendants is rejected while direct self-loops and duplicate children retain their existing behavior. Done means indirect cycles cannot be created and recursive collection access remains bounded.