feat: Add Neo4j Graph Backend With Hop-Capped Follow Walks - #13
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Model the activity feed as a labeled property graph: User and Post nodes, directed FOLLOWS and AUTHORED edges, unique constraints, and the path queries (shortestPath, friend-of-friend, common neighbors) that the tabular stores express as recursive CTEs.
Parse neo4j-driver ConstraintValidationFailed messages (no .constraint), cap shortest FOLLOWS BFS at 16 hops with a 17-edge chain, and pin the driver Cypher strings including WHERE a <> b on follow.
ThomasHartDev
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FOLLOWS and AUTHORED are directed edges on User and Post nodes. Neo4jStore walks an in-process MemoryGraph (neighbor lists and BFS), not a neo4j-driver session. Unique handles are CREATE CONSTRAINT. shortestFollowPath is BFS capped at 16 hops. recommendFollows is a 2-hop friend-of-friend score. CYPHER in src/neo4j/cypher.ts is the matching query catalog, pinned in tests, including WHERE a <> b on follow so a pasted MERGE cannot create a self-loop.
The contract runs without a daemon.
Closes #12