chore: harden knowledge graph storage and clear common clippy warnings

Enforce stable 1:1 entity embeddings, relationship endpoint auth, and
user-scoped deletes; align schemas/migrations and resolve common crate
clippy findings.

common/migrations/20260528_000002_knowledge_graph_storage_hardening.surql

@@ -0,0 +1,33 @@
+-- Harden knowledge entity embeddings and graph storage invariants.
+
+DEFINE FIELD IF NOT EXISTS source_id ON knowledge_entity_embedding TYPE string;
+
+-- Backfill denormalized source_id from the linked entity.
+FOR $emb IN (SELECT * FROM knowledge_entity_embedding WHERE source_id = NONE OR source_id = '') {
+    LET $entity = (SELECT source_id FROM $emb.entity_id)[0];
+    IF $entity != NONE {
+        UPDATE $emb.id SET source_id = $entity.source_id;
+    }
+};
+
+-- Re-key embeddings so record id matches entity id (stable 1:1 identity).
+FOR $emb IN (SELECT * FROM knowledge_entity_embedding) {
+    LET $entity_key = record::id($emb.entity_id);
+    LET $canonical = type::thing('knowledge_entity_embedding', $entity_key);
+    IF $emb.id != $canonical {
+        UPSERT $canonical CONTENT {
+            entity_id: $emb.entity_id,
+            embedding: $emb.embedding,
+            user_id: $emb.user_id,
+            source_id: $emb.source_id,
+            created_at: $emb.created_at,
+            updated_at: $emb.updated_at
+        };
+        DELETE $emb.id;
+    }
+};
+
+REMOVE INDEX IF EXISTS knowledge_entity_embedding_entity_id_idx ON knowledge_entity_embedding;
+DEFINE INDEX IF NOT EXISTS knowledge_entity_embedding_entity_id_idx ON knowledge_entity_embedding FIELDS entity_id UNIQUE;
+DEFINE INDEX IF NOT EXISTS knowledge_entity_embedding_source_id_idx ON knowledge_entity_embedding FIELDS source_id;
+DEFINE INDEX IF NOT EXISTS knowledge_entity_user_source_idx ON knowledge_entity FIELDS user_id, source_id;