feat: add channel frequency labels to metrics

[itsDNNS]

Summary

• add frequency labels to channel-level Prometheus metrics
• normalize channel frequencies to a stable MHz label format where possible
• extend Prometheus tests to cover downstream and upstream frequency labels

Problem

Issue #227 requests the actual channel frequency on the metrics endpoint so downstream analysis can correlate metrics without relying on channel_id alone.

Current metrics only expose channel_id, for example:
docsight_upstream_modulation{channel_id="2"} 64

Fix

All channel-level Prometheus metric families now emit a shared label set:

• channel_id
• frequency when the channel exposes one

Frequency values are normalized for label stability:

• raw Hz-like values such as 30000000 become 30
• values like 44 MHz become 44
• non-numeric values fall back to the original text

Verification

• pytest -q tests/test_prometheus.py
• result: 54 passed

Closes #227.

[itsDNNS]

Note: frequency is emitted only when the channel exposes one. This keeps the metrics backward-compatible, but dashboards grouping by frequency should account for channels where that label may be absent.