Feat: Migrate Thresholds to Benchmarking model

[svenvg93]

📃 Description

Extends the benchmark/threshold system to close three specific asks — packet loss support, relative (ISP-baseline-percentage) thresholds, and recording an ISP baseline to compare against. The old flat ThresholdSettings (a single global set of absolute download/upload/ping values) is replaced with a Benchmark model — one row per metric (download, upload, ping, packet loss) — each configurable as either an absolute value or a percentage of a recorded ISP baseline.

Closes #2155, #908, #1943.

🪵 Changelog

➕ Added

• Packet loss as a fourth benchmarkable metric, alongside download/upload/ping
• Relative benchmark type — compare a result against a percentage of a recorded ISP baseline value, available for all four metrics
• Benchmarks admin resource (Settings → Benchmarks) — a table of the four metrics with an inline enable toggle and an edit-in-modal form for absolute/relative configuration
• Packet loss pass/fail indicator on the dashboard's latest-result stat card, matching download/upload/ping

✏️ Changed

• Notification settings: *_on_threshold_failure renamed to *_on_benchmark_failure (existing preferences carry over automatically)

🗑️ Removed

• ThresholdSettings, the old Thresholds settings page, and the THRESHOLD_* env vars/config keys
• App\Helpers\Benchmark (logic moved onto the new Benchmark model)

📷 Screenshots