feat: persist peer session state across disconnects ("welcome back" on reconnect)

Save groups, profile, visibility, summary, display name, and cumulative
stats to a new mesh.peer_state table on disconnect. On reconnect (same
meshId + memberId), restore them automatically — hello groups take
precedence over stored groups if provided. Broadcast peer_returned
system event with last-seen time and summary to other peers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/db/migrations/0014_peer-state-persistence.sql

@@ -0,0 +1,16 @@
+-- Peer session persistence: save state on disconnect, restore on reconnect.
+CREATE TABLE IF NOT EXISTS mesh.peer_state (
+  id TEXT PRIMARY KEY NOT NULL,
+  mesh_id TEXT NOT NULL REFERENCES mesh.mesh(id) ON DELETE CASCADE ON UPDATE CASCADE,
+  member_id TEXT NOT NULL REFERENCES mesh.member(id) ON DELETE CASCADE ON UPDATE CASCADE,
+  groups JSONB DEFAULT '[]',
+  profile JSONB DEFAULT '{}',
+  visible BOOLEAN NOT NULL DEFAULT true,
+  last_summary TEXT,
+  last_display_name TEXT,
+  cumulative_stats JSONB DEFAULT '{"messagesIn":0,"messagesOut":0,"toolCalls":0,"errors":0}',
+  last_seen_at TIMESTAMP,
+  created_at TIMESTAMP NOT NULL DEFAULT now(),
+  updated_at TIMESTAMP NOT NULL DEFAULT now(),
+  CONSTRAINT peer_state_mesh_member_idx UNIQUE (mesh_id, member_id)
+);