fix: re-enable CI lint job and fix ~65 lint errors (partial)

Re-enable the golangci-lint job in CI (disabled Oct 2025), update to
Go 1.25 and golangci-lint-action v7. Fix errcheck, gosimple S1009,
staticcheck SA4031 and SA2001 errors across 54 files. Remaining ~39
lint errors will be addressed in follow-up commits.

server/channelserver/testhelpers_db.go

@@ -170,11 +170,11 @@ func applyPatchSchemas(t *testing.T, db *sqlx.DB, projectRoot string) {
 
 		_, err = tx.Exec(string(patchSQL))
 		if err != nil {
-			tx.Rollback()
+			_ = tx.Rollback()
 			t.Logf("Warning: Failed to apply patch %s: %v", filename, err)
 			// Continue with other patches even if one fails
 		} else {
-			tx.Commit()
+			_ = tx.Commit()
 		}
 	}
 }