D7: Generic hwmon temperature-sensor collector (enumerate all sensors)

[LarsLaskowski]

Epic: More Metrics

Why

Today temperature.go reports only the CPU/SoC thermal zone (plus the same-die "GPU" value via vcgencmd). But a Pi can expose several genuinely separate temperature sensors through the kernel hwmon subsystem — depending on the board and attached hardware: the SoC sensor itself, a PoE-HAT fan controller, NVMe/SSD drives (notably on Pi 5), and user-attached I²C/1-Wire sensors (e.g. DS18B20, BME280, LM75). The fixed thermal-zone lookup can't see these. A generic hwmon collector surfaces all present sensors automatically, with no extra dependency and no vcgencmd.

Scope

Add a collector that walks /sys/class/hwmon/hwmon*/:

• For each temp*_input file (millidegrees Celsius, integer), read the value.
• Read the sibling temp*_label for a human-readable name when present; fall back to the chip name file + channel index otherwise.
• Return a list of {chip, label, celsius} entries.

This is pure sysfs parsing, fixture-testable, and follows the existing collector pattern. It naturally includes the SoC sensor and any attached/onboard sensors. Keep the parser independent of real /sys access (feed synthetic hwmon trees in tests).

Reuse & touch points

• New internal/collector/hwmon.go + hwmon_test.go, following the collector + fixture-test pattern (see cpu.go / cpu_test.go, and the millidegree parsing in temperature.go readThermalZoneMilliC, line 58).
• Reuse testhelpers_test.go (writeTempFile) to build synthetic hwmon*/temp*_input + temp*_label trees.
• New Sensors []TemperatureSensor field in internal/collector/types.go; assemble in collector.go.
• Optionally show extra sensors in the Temperature card (internal/web/assets/).

API & config impact

Additive sensors[] array in the snapshot (omitempty when none); no breaking change. Consider a hwmon_enabled config toggle. Update docs/API.md.

Acceptance criteria

• A synthetic hwmon tree with multiple chips/channels parses to the expected {chip, label, celsius} list, including correct millidegree→°C conversion.
• Missing temp*_label falls back to chip name + index.
• Off-Pi / no hwmon directory: empty list, no error.

Relationship to existing temperature collector & D6

• The existing dedicated CPU-temperature reading (temperature.go) stays as the primary, clearly-labelled value for the dashboard gauge; this collector is additive breadth, not a replacement (avoids changing the existing temperature JSON shape — v1 stability).
• The Pi PMIC sensor is not exposed via hwmon on Raspberry Pi OS, so it is out of scope here and handled separately in D6 (vcgencmd measure_temp pmic). D6 and D7 are complementary.