refactor: rename memory_entity→knowledge_node, knowledge graph edge→knowledge_edge

Resolves table name collision between:
- memory_edge (provenance DAG: child_sha/parent_sha) — KEPT
- knowledge_edge (knowledge graph: source_id/target_id) — NEW NAME

Changes:
- memory_entity → knowledge_node (all .rs + migrations 003-009)
- knowledge graph memory_edge → knowledge_edge
- memory_entity_version → knowledge_node_version
- memory_edge_version → knowledge_edge_version
- Added knowledge_node + knowledge_edge to init_schema()
- Converted versioning.rs from sqlx::query_as! to runtime queries
  (avoids stale sqlx offline cache dependency)
- Fixed UUID cast: $1::UUID for String→UUID column binds
- Fixed column names: source_entity_id→source_id, target_entity_id→target_id

Production DB: knowledge_node + knowledge_edge tables created,
memory_entity VIEW points to knowledge_node for backward compat.

tests/it_phase7_gate.rs

@@ -50,8 +50,8 @@ async fn test_phase7_composition_gate() {
 
 // T7.1: Schema Checks
 fn assert_tables_exist() {
-    // Verify memory_entity_version table
-    // Verify memory_edge_version table
+    // Verify knowledge_node_version table
+    // Verify knowledge_edge_version table
     // Verify index coverage
 }
 
@@ -107,7 +107,7 @@ async fn test_edge_diff_endpoint() {
 async fn test_audit_logger_log_entity() {
     // Create entity snapshot
     // Call audit_logger.log_entity()
-    // Verify record inserted in memory_entity_version
+    // Verify record inserted in knowledge_node_version
     // Verify changed_by populated from JWT sub
     // Verify fields_changed array computed
 }
@@ -120,9 +120,9 @@ async fn test_audit_logger_history() {
 }
 
 async fn test_audit_logger_immutability() {
-    // Try to UPDATE memory_entity_version
+    // Try to UPDATE knowledge_node_version
     // Should fail (trigger fires)
-    // Try to DELETE memory_entity_version
+    // Try to DELETE knowledge_node_version
     // Should fail (trigger fires)
     // Verify log is append-only
 }