feat: Add Browserless rendering backend for image exports

[jordanm-posthog]

Problem

Move image export to Browserless behind a feature flag

Changes

Add a Playwright/browserless rendering backend for PNG image exports (insights, dashboards, replays, heatmaps) alongside the existing Selenium path.

How did you test this code?

Locally, new tests

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Automatic notifications

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[greptile-apps]

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 2
posthog/settings/exports.py:3-4
Inconsistent env var naming will confuse operators. Two of the four settings read from `HEATMAP_BROWSERLESS_*` env vars (which belong to the Heatmap feature), while the two timeout settings read from `BROWSERLESS_*` env vars. A deployer who reads `BROWSERLESS_SESSION_TIMEOUT_MS` and logically tries to set `BROWSERLESS_CDP_URL` will have no effect — the actual env var that matters is `HEATMAP_BROWSERLESS_URL`. If re-using the existing Heatmap instance is intentional, the variable names or comments should make that explicit; otherwise the env var names should match their Python setting names.

```suggestion
BROWSERLESS_CDP_URL: str = get_from_env("BROWSERLESS_CDP_URL", "")
BROWSERLESS_TOKEN: str = get_from_env("BROWSERLESS_TOKEN", "")
```

### Issue 2 of 2
posthog/tasks/exports/image_exporter.py:597
The page-load timeout re-raise drops the exception cause chain. The original `PlaywrightTimeoutError` has already been sent to Sentry via `capture_exception`, but Python's chained-exception context is lost, which makes it harder to trace the root error in local logs or wherever the re-raised exception is caught upstream. Adding `from e` preserves the chain at zero cost.

```suggestion
                raise PlaywrightTimeoutError("Timeout while waiting for the page to load") from e
```

_{Reviews (1): Last reviewed commit: "Add Browserless rendering backend for im..." | Re-trigger Greptile}

[deployment-status-posthog]

Deploy status

Environment	Status	Deployed At	Workflow
dev	✅ Deployed	2026-06-01 15:00 UTC	Run
prod-us	✅ Deployed	2026-06-01 15:26 UTC	Run
prod-eu	✅ Deployed	2026-06-01 15:32 UTC	Run