docs(controlplane): note why PostgresLock instead of NATS for coordination

NATS is optional infrastructure; Postgres is the only piece every
control-plane deployment has. Advisory locks let us coordinate
background jobs across replicas without adding a new dependency.

Assisted-by: Claude Code
Signed-off-by: Miguel Martinez Trivino <miguel@chainloop.dev>

Chainloop-Trace-Sessions: 052e8b56-72b5-4c6c-8d82-ab2d00728889

Author: migmartri

app/controlplane/pkg/data/lock.go

@@ -34,6 +34,12 @@ const advisoryUnlockTimeout = 5 * time.Second
 // PostgresLock implements biz.DistributedLock using Postgres session-level
 // advisory locks (pg_try_advisory_lock / pg_advisory_unlock).
 //
+// Postgres is the only piece of infrastructure that's mandatory for the
+// control plane — NATS is optional (used for distributed caches when
+// present). Using advisory locks lets us coordinate background jobs
+// across replicas without adding a new dependency (a NATS KV lease or a
+// dedicated queue) that wouldn't be available in every deployment.
+//
 // Each lock holds a dedicated connection for its lifetime; releasing the
 // lock returns the connection to the pool. If the pod crashes mid-run the
 // connection drops and Postgres releases the lock automatically.