Fix multi-WAN speed test reporting (issue #841)

Speed tests were not being reported correctly for multi-WAN setups
because the device-level speedtest-status field was returning zeros.
The data has moved to a new aggregated dashboard API endpoint.

Changes:
- Add GetSpeedTests() and GetSiteSpeedTests() methods to fetch from
  /v2/api/site/{site}/aggregated-dashboard endpoint
- Create SpeedTestResult data structures to capture per-WAN metrics
- Update Prometheus exporter with new speedtest_* metrics per interface
- Update InfluxDB exporter to write speedtest measurements per WAN
- Update Datadog exporter with unifi.speedtest.* metrics per WAN
- Update metrics collection to include speed test data for all sites

Metrics now include labels/tags for:
- wan_interface: Physical interface (eth8, eth9, etc.)
- wan_group: Logical WAN name (WAN, WAN2, etc.)
- site_name: Site identifier
- source: Controller URL

Gracefully handles older controllers without the new API endpoint.

Fixes #841

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

go.sum

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