`push_to_falkordb` and `push_to_neo4j` upsert every node with
`MERGE (n:<Label> {id: $id}) SET n += $props`, which matches on (label, id).
Neither path created an index on that pattern first, so the engine had to
scan every node already carrying the label to decide whether the node is
already there. Each upsert then costs O(nodes with that label) and a full push
degrades to O(n^2): on the graph in #3804 (167,428 nodes / 209,215 edges
across 6 labels) throughput collapsed from ~10,000 node upserts/60s to
~100 upserts/300s partway through, with the database at ~97% CPU.
Create one index per distinct label up front, before the upsert loop. The
label set is derived exactly as the push loops derive it, so every label a
MERGE matches against is covered. Neo4j uses IF NOT EXISTS to stay idempotent
across re-pushes; FalkorDB's CREATE INDEX has no such clause, so a re-push
against an already-indexed graph is tolerated rather than failing an
otherwise valid push.
Refs #3804
Co-Authored-By: opencode space-bunny-free <noreply@opencode.ai>
(cherry picked from commit a3275e2c97)
