Cover AC#1-4 (leaf=direct, distinct overlap counted once, full descendant
depth), REQ-THEMEN-05 (empty subtree absent), REQ-THEMEN-06 (cycle terminates
via the 50-level guard) and AC#7 (rollup equals distinct documents found by the
real tag-search expansion — count↔destination parity). Testcontainers
postgres:16-alpine since the recursive CTE + COUNT(DISTINCT) needs real PG.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>