experiment: graph explorer API (neighborhood, shortest path, reindex)

[SHAcollision]

experiment: backend half of the graph explorer. Use together with the pubky-app branch experiment/graph-viz (frontend PR: pubky/pubky-app#2138).

What it adds

• GET /v0/graph/{kind}/{id} with kind in user|post|tag: a typed node-link neighborhood around a center entity, painted in one request. Params: depth (1..2, user centers only), limit (1..50, default 30), kinds (csv filter). Node ids are kind-prefixed (user:{pubky}, post:{author}:{post_id}, tag:{label}); FOLLOWS/TAGGED edges carry indexed_at so the client can time-travel.
• GET /v0/graph/path/{from}/{to}: undirected shortest FOLLOWS path, max 6 hops, nodes path-ordered. Distinct 404s for unknown users vs no path.
• nexusd db reindex: rebuilds the whole Redis index from the Neo4j graph (flush, recreate the RediSearch post-content FT index, resync). Needed once after restoring a graph backup, and reusable as an operational tool.

Design notes

• Neighborhood queries are single COLLECTed-row Cypher with per-class caps (hop-2 has a global DISTINCT cap of 120), anchored subqueries (no full TAGGED scans, verified with EXPLAIN), and deleted-user filtering.
• Params validate through the house idioms: BoundedLimit, a GraphDepth newtype in the WotDepth style, GraphKinds with its own CSV Deserialize, Path<PubkyId> extraction. Invalid input is a 400, not a silent clamp.
• Shared Cypher fragment helpers (user_projection, post_fields, not_deleted on USER_DELETED_SENTINEL) keep the projections in one place.
• MockDb::sync_redis and db reindex now share one rebuild(); this fixes a real gap where a reindex skipped the FT index and post search came back empty. The flush is FLUSHALL ASYNC so production-sized indexes do not outlive the client timeout.

Tests

14 integration tests (nexus-webapi/tests/graph/) over the mock dataset: shapes for all three center kinds, depth/limit/kinds behavior, edge-endpoint closure, path ordering, and the full 4xx surface. Serde unit tests for the new deserializers. Full suite green: 625 passed.

We probably will not merge this as-is; it exists so the team can explore the feature end to end.