Switch handler test databases from file-backed SQLite (with
WAL mode and temp dir) to in-memory SQLite. This eliminates
filesystem I/O overhead per test, cutting handler test time
roughly in half.
Before: ~20s in Docker (ARM), exceeding 30s timeout on CI
After: ~10s in Docker (ARM), well within 30s timeout
Also consolidate the two test server constructors
(newTestServer and newTestServerWithOper) into a shared
newTestServerWith helper, removing ~50 lines of duplicate
setup code.
closes#90