Prevent missed updates by using stable scheduler checkpoints #13

Problem

The checkpoint is updated to DateTimeOffset.UtcNow after external API calls and message creation:

• Checkpoint update

An event created after the API response but before the checkpoint update can be permanently skipped.

External clients also read only the first page:

• GitHub request
• StackOverflow request

Proposed solution

• Capture checkStartedAt before requesting the external API.
• Persist checkStartedAt only after detected events have been stored.
• Store stable source event identifiers where available.
• Handle external API pagination until the previous checkpoint is reached.
• Deduplicate source events by provider and external event ID.

Acceptance criteria

• Events created during a check are detected during the next check.
• A checkpoint advances only after events are persisted successfully.
• GitHub and StackOverflow pagination is supported.
• Repeated API pages do not create duplicate events.
• Tests cover events created between request start and checkpoint commit.