Fix duplicate event subscriptions on retry - #87

Summary

subscribeToBeaconEvents subscribed topics one at a time and gave up on the first failure, leaving earlier topics subscribed. The retry loop then resubscribed the whole list every two seconds, so topics before a bad one built up duplicate live subscriptions forever (each one an independently reconnecting stream, so handlers fire once per duplicate) while topics after it never subscribed at all.

Fix tracks already-subscribed topics and skips them on retry, and logs and skips a topic that fails instead of aborting the rest of the list.

Test plan

• Added tests in pkg/beacon/subscriptions_test.go: one with a permanently bad topic in the middle of the list confirming good topics before and after it each subscribe exactly once, one confirming the happy path returns once all topics subscribe
• Confirmed the first test fails against the old code and passes against the fix
• go build ./..., go vet ./..., go test -race ./... all green