Add public bounded flush API

[marandaneto]

Summary

Add a public bounded flush API.

Why

sdk-specs marks flush as applicable to both client and server SDKs. Backend users need deterministic delivery before process/request boundaries, tests, scripts, and serverless exits.

Current status

PostHog.Sender flushes on batch size, timer, and GenServer termination, but there is no public flush API. Python has a related issue about adding a max timeout to flush: PostHog/posthog-python#79.

Suggested scope

• Add a public PostHog.flush/1 or PostHog.flush/2 accepting a named instance and timeout.
• Flush all sender queues immediately without waiting for threshold/timer.
• Return safely when queues are empty.
• Bound the wait with a timeout and return/report status rather than blocking indefinitely.
• Preserve retryable failed events for future retry once retry queue support exists.
• Add tests and docs.

References

• https://github.com/PostHog/sdk-specs/blob/main/openspec/specs/flush/spec.md