Fix race in primary election.

[NullHypothesis]

The methods Init and PostInit each decided primary-vs-standby by reading the Consul "cluster initialized" flag and acting on it separately, with no atomicity between the two. When nodes booted close together, more than one could see the flag unset and independently self-initialize as primary, leaving a node with a fresh, credential-less local database that then failed forever trying to register as a standby against the real primary (repmgr/flypgadmin roles don't exist).

Replace both checks with Store.TryClaimPrimary, a single atomic Consul CAS (ModifyIndex: 0) keyed by machine ID, so exactly one node ever wins the claim, and a node retrying after its own crash can tell it already won rather than deferring to itself as "someone else."

[NullHypothesis]

I'm going to ignore the "cyclomatic complexity" complaint because the PR isn't making anything worse. In fact, it's removing a bit of code.

[NullHypothesis]

Here's how I tested this:

This command currently fails for me. The leader passes all its health checks and thinks it's the leader but the other two machines (or at least one other machine) often disagree.

fly pg create --flex --name myapp-bad --initial-cluster-size 3 --vm-size shared-cpu-1x --volume-size 1

Using the patched postgres-flex image (I built it locally and manually uploaded to our registry), the command succeeds:

fly pg create --flex --name myapp-good --initial-cluster-size 3 --vm-size shared-cpu-1x --volume-size 1 --image-ref registry.fly.io/philipp-test-pg:my-fix-test