Feat: Add prometheus remote write support

[svenvg93]

Important

should be updated and verified after #2802

📃 Description

Adds Prometheus Remote Write support, allowing speedtest metrics to be pushed directly to a remote write endpoint after each speedtest completes. Compatible with Prometheus, VictoriaMetrics, Grafana Cloud (Mimir), and Cortex.

The Prometheus settings tab has been reorganised into two sections: Scrape Endpoint (existing pull-based setup) and Remote Write (new push-based setup).

📖 Documentation

TBD

🪵 Changelog

➕ Added

• Prometheus Remote Write support — metrics are pushed to a configurable endpoint after each speedtest
• PrometheusRemoteWriteService to build and send Protobuf-encoded payloads compressed with Snappy
• App\Jobs\Prometheus\WriteResult job to dispatch remote write asynchronously via the queue
• Settings for remote write: URL, optional username/password, and enabled toggle
• Settings migration to add prometheus_remote_write_* keys to the dataintegration group
• Feature tests for remote write covering enabled/disabled state, authentication, queue dispatch, and HTTP failures

➕ New dependency

The Prometheus Remote Write protocol (spec) mandates that the protobuf payload is compressed with Snappy before sending. The Content-Encoding: snappy header is required — endpoints that implement the spec strictly (including self-hosted Prometheus with --enable-feature=remote-write-receiver) will reject requests with any other encoding, returning a 415 Unsupported Media Type error.

PHP has no built-in Snappy support (unlike gzip via gzencode()), and the only alternative — the ext-snappy PECL extension — requires compiling a native PHP extension into the Docker image, which would complicate the deployment significantly.

flow-php/snappy (^0.38.0) is a pure-PHP implementation of the Snappy algorithm that works without any PHP extension.

✏️ Changed

• Prometheus settings tab reorganised into Scrape Endpoint and Remote Write sections
• PrometheusMetricsService and PrometheusRemoteWriteService moved to App\Services\Prometheus\ to align with the existing App\Jobs\Prometheus\ namespace
• ProcessSpeedtestDataIntegrations listener now dispatches WriteResult job when remote write is enabled