fix: add nolint:errcheck comments for intentionally unchecked errors

Add //nolint:errcheck comments to intentionally unchecked error returns:

- defer X.Close() calls: errors from closing read-only resources, rows
  after iteration, files, response bodies, and gzip readers are not
  actionable and don't affect correctness

- defer tx.Rollback(): standard Go pattern where rollback after
  successful commit returns an error, which is expected behavior

- defer stmt.Close(): statements are closed with their transactions

- Cleanup operations: DeleteRevision on failure and os.RemoveAll for
  temp directories are best-effort cleanup

- HTTP response encoding: if JSON encoding fails at response time,
  there's nothing useful we can do

- Test/benchmark code: unchecked errors in test setup/cleanup where
  failures will surface through test assertions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/database/database_test.go

@@ -27,7 +27,7 @@ func runStoreTests(t *testing.T, newStore func(t *testing.T) Store) {
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			store := newStore(t)
-			defer store.Close()
+			defer store.Close() //nolint:errcheck // test cleanup
 			tt.test(t, store)
 		})
 	}