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.

crates/mem-core/src/edge.rs

@@ -51,8 +51,8 @@ pub struct Edge {
     pub project_id: String,
     
     // Relationship
-    pub source_entity_id: String,  // FK to memory_entity
-    pub target_entity_id: String,  // FK to memory_entity
+    pub source_entity_id: String,  // FK to knowledge_node
+    pub target_entity_id: String,  // FK to knowledge_node
     pub relation_type: String,
     pub fact: String,
     pub fact_embedding: Option<Vec<f32>>,
@@ -71,7 +71,7 @@ pub struct Edge {
     
     // Provenance
     pub source_episode_id: Option<i64>,  // FK to memory_node
-    pub invalidated_by: Option<String>,  // FK to memory_edge.id
+    pub invalidated_by: Option<String>,  // FK to knowledge_edge.id
     
     // Contradiction handling
     pub contradiction_status: ContradictionStatus,