fix: check all Close() return values for errcheck lint

Add explicit error discards (_ =) for Close() calls on network
connections, SQL rows, and file handles across 28 files. Also add
.golangci.yml with standard linter defaults to match CI configuration.

.golangci.yml

@@ -0,0 +1,7 @@
+version: "2"
+
+run:
+  timeout: 5m
+
+linters:
+  default: standard