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.

common/token/token_test.go

@@ -334,6 +334,7 @@ func TestGenerate_ConsistentCharacterSet(t *testing.T) {
 
 func TestRNG_Type(t *testing.T) {
 	// Verify RNG is of type *SafeRand
-	var _ *SafeRand = RNG
-	var _ *SafeRand = NewSafeRand()
+	var _ = (*SafeRand)(nil)
+	_ = RNG
+	_ = NewSafeRand()
 }