Unify NostrUser and Author nodes; add migrations support

Merged 'Author' nodes into 'NostrUser' for unified identity tracking and social graph representation. Introduced migrations framework to handle schema changes, including retroactive updates for existing relationships and constraints. Updated tests, schema definitions, and documentation to reflect these changes.

pkg/neo4j/neo4j.go

@@ -135,6 +135,9 @@ func NewWithConfig(
 		return
 	}
 
+	// Run database migrations (e.g., Author -> NostrUser consolidation)
+	n.RunMigrations()
+
 	// Initialize serial counter
 	if err = n.initSerialCounter(); chk.E(err) {
 		return
@@ -298,10 +301,8 @@ func (n *N) EventIdsBySerial(start uint64, count int) (
 	return
 }
 
-// RunMigrations runs database migrations (no-op for neo4j)
-func (n *N) RunMigrations() {
-	// No-op for neo4j
-}
+// RunMigrations is implemented in migrations.go
+// It handles schema migrations like the Author -> NostrUser consolidation
 
 // Ready returns a channel that closes when the database is ready to serve requests.
 // This allows callers to wait for database warmup to complete.