Now that the Molecule-AI org has two self-hosted Apple-silicon runners
(`hongming-m1-mini` + `hongming-m1-mini-2`) servicing the same label set,
two CI runs could execute the e2e-api job concurrently. Each run starts
fixed-name docker containers (`molecule-ci-postgres`, `molecule-ci-redis`)
bound to host ports 15432/16379 — a collision means the second run fails
with "container name already in use" or "port already in use".
Adds a workflow-level `concurrency: e2e-api` group to the job so GitHub
Actions serializes e2e-api executions globally regardless of which runner
picks them up. `cancel-in-progress: false` ensures later runs queue
rather than cancelling the in-flight one (we want every PR's e2e check
to actually execute, not get skipped by a newer push).
Tradeoff: e2e-api is now effectively single-threaded across the whole
org. Measured duration is ~1-2 min per run, so the added serialization
latency is small relative to total CI wall time. All other jobs still
parallelize across both runners.
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