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/save-event_test.go

@@ -542,7 +542,7 @@ func TestSaveEvent_ETagReference(t *testing.T) {
 
 	// Verify MENTIONS relationship was also created for the p-tag
 	mentionsCypher := `
-		MATCH (reply:Event {id: $replyId})-[:MENTIONS]->(author:Author {pubkey: $authorPubkey})
+		MATCH (reply:Event {id: $replyId})-[:MENTIONS]->(author:NostrUser {pubkey: $authorPubkey})
 		RETURN author.pubkey AS pubkey
 	`
 	mentionsParams := map[string]any{