feat: swarm membership discovery via peer-list gossip

Refs #1

- /v1/ping now advertises the node's known peers (peer_id, addr, name)
- Run sweeps static KNOX_PEERS + persisted discovered peers, enqueueing
  newly-learned addresses for direct sweeps (membership-only relay; no
  observation relay)
- db: ShareablePeers, SwarmPeerAddrs, MergePeer (cursor-preserving
  discovery upsert), MaxHCLForNode
- integration test: a node configured with a single seed discovers and
  pulls from other swarm members without direct configuration

internal/db/schema.go

@@ -105,8 +105,6 @@ CREATE TABLE IF NOT EXISTS threads (
     cluster_key TEXT
 );
 
-CREATE UNIQUE INDEX IF NOT EXISTS idx_threads_cluster ON threads(cluster_key) WHERE cluster_key IS NOT NULL AND cluster_key != '';
-
 -- SETTINGS: key-value store for runtime configuration
 CREATE TABLE IF NOT EXISTS settings (
     key   TEXT PRIMARY KEY,