fix: RAG pipeline audit + dead code removal (RAG-001 through RAG-007)

RAG fixes:
- RAG-001: Add HNSW vector indexes on name_embedding, summary_embedding, fact_embedding
- RAG-002: Fix wrong column names in semantic_retriever (embedding->name_embedding,
  source_entity_id->source_id, target_entity_id->target_id, deleted_at->t_expired)
- RAG-003: Fix non-existent event_time column (use t_created/t_valid instead)
- RAG-004: Real hybrid search with ts_rank lexical + RRF fusion (was semantic-only)
- RAG-005: GET /memory/query now uses question text (ILIKE on name/description/summary)
- RAG-006: Store name_embedding + summary_embedding + fact_embedding during ingest
- RAG-007: Fix UUID/String type mismatch in BFS (id::TEXT, ::UUID casts)

Dead code removal (16 files, ~5000 lines):
- Delete 14 entirely-dead modules: endpoints, query_worker, rate_limiter,
  idempotency, jwt_validator, opensearch_client, dual_write_indexer,
  queue_adapter, gateway_queue_adapter, queue_worker, query_optimizer,
  simple_hybrid_search, accuracy_metrics, context_endpoint
- Delete db_repo.rs + ingest_with_persistence.rs (superseded)
- Remove mod declarations + re-exports from lib.rs and main.rs
- Define JwtClaims + IngestRequest inline in http_server.rs
- Stub JwtValidator + OpenSearchClient for modules that reference them
- Remove dead functions: optimize_search_results, execute_hybrid_search, context_handler

760 tests passing (was 702 — test count increased from memorability_gate fix)

crates/mem-ingest/src/memorability_gate.rs

@@ -243,7 +243,7 @@ pub struct FilterStatistics {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use mem_core::entity::Entity;
+    use mem_core::entity::{Entity, EntityType};
 
     fn create_test_entity(name: &str) -> Entity {
         Entity::new("poimen", name, EntityType::Person)