Capture V1 protocol now available; Bearer auth and per-event results

Minor changes

• d459b57 Add an opt-in capture_mode for the Capture V1 ingestion protocol (POST /i/v1/analytics/events). Set capture_mode="v1" on the client (or the POSTHOG_CAPTURE_MODE=v1 environment variable) to use Bearer auth, per-event results, and partial retry. Defaults to "v0" (the legacy /batch/ endpoint), so existing setups are unaffected.

  When using capture_mode="v1", request bodies can be compressed via capture_compression (or POSTHOG_CAPTURE_COMPRESSION): "gzip", "deflate", "zstd" (requires the optional posthog[zstd] extra), or "none" (default). The legacy gzip=True flag is honored as a fallback.

  Per-event server verdicts are surfaced through the existing on_error handler: events the backend explicitly drops, or fails to accept after retries, raise a CaptureV1Error carrying the affected event UUIDs — so a rejection is never silently lost, even when the HTTP request itself succeeded. — Thanks @eli-r-ph for your first contribution 🎉!