Capture V1 endpoint now opt-in via env var…

#4105 203284a Thanks @eli-r-ph! - Add opt-in Capture V1 support. Set the POSTHOG_CAPTURE_MODE=v1 environment variable to submit analytics events to the Capture V1 endpoint (/i/v1/analytics/events) instead of the legacy /batch/ endpoint, on both the batched and immediate send paths. The default remains v0, so existing behavior is unchanged unless you opt in. Opt-in is env-var-only during the transition (no public option), so nothing on the API surface has to be removed when v1 later becomes the default.

Capture V1 uses Bearer auth, lifts legacy $-sentinel properties into a typed options object, and does per-event partial retry with exponential backoff clamped against Retry-After. Dropped and undelivered events are surfaced on the client error channel as a CaptureV1Error. $ai_* events continue to use the legacy submitter for now, regardless of the capture mode.

In v1 mode, $ai_* events are routed to an isolated in-memory queue and flushed independently of the Capture V1 queue, so the two transports never share a batch and a failure on one cannot re-send events already accepted on the other. Each queue keeps its own retry/durability semantics: the legacy queue re-queues on network failure (retrying on later flushes), while the V1 queue exhausts the sender's own attempt budget per cycle and then surfaces the failure rather than re-queuing. (2026-07-11)

Capture V1 endpoint now opt-in via env var