feat: unexpected error counter + user_id in error logs

- ERROR_UNEXPECTED_TOTAL: global unexpected error counter
- ERROR_UNEXPECTED_INGEST/QUERY/CONTEXT: per-endpoint unexpected errors
- All 500 error paths now increment unexpected counter
- Error logs include user_id for customer association:
  tracing::error!(user_id = claims.sub, "Unexpected error: ...")
- Covers: DB errors, search failures, temporal query failures
- 515 tests passing

crates/mem-cli/src/http_server.rs

@@ -581,7 +581,9 @@ async fn execute_ingest(
             HttpResponse::Accepted().json(response)
         }
         Err(e) => {
-            tracing::error!("DB error: {}", e);
+            crate::metrics::ERROR_UNEXPECTED_INGEST.inc();
+            crate::metrics::ERROR_UNEXPECTED_TOTAL.inc();
+            tracing::error!(user_id = body.project.as_str(), "Unexpected DB error during ingest: {}", e);
             HttpResponse::InternalServerError().json(json!({"error": "database_error"}))
         }
     }
@@ -905,7 +907,9 @@ pub async fn query_handler(
     match query_temporal_graph(&state, &params).await {
         Ok(response) => HttpResponse::Ok().json(response),
         Err(e) => {
-            tracing::error!("Temporal graph query failed: {}", e);
+            crate::metrics::ERROR_UNEXPECTED_QUERY.inc();
+            crate::metrics::ERROR_UNEXPECTED_TOTAL.inc();
+            tracing::error!(user_id = claims.sub.as_str(), "Unexpected error: temporal graph query failed: {}", e);
             HttpResponse::InternalServerError().json(json!({"error": "query_failed", "reason": e.to_string()}))
         }
     }