Add second analog input for optional sensor sampling #5

Summary

Add support for a second input channel that can be sampled like the existing DHT extra-sensor readings, but as a simple analog value. Stephen described this as high importance for expanding WaterPAL beyond the current pump-use input.

Use cases

Potential sensors / applications mentioned:

• IR sensors for solar pumps
• Grease detection
• Handle movement detection
• Other plug-in analog Arduino-style modules

Example sensor links from the discussion:

• https://a.co/d/0a5c1s5j
• https://www.amazon.com/dp/B07W97H2WS

Stephen confirmed analog is enough, and thresholding can happen on the database side.

Requested behavior

• Sample one additional analog input on a regular schedule, similar to how DHT temperature / humidity readings are sampled.
• Store and report the analog readings alongside existing extra sensor values where practical.
• Do not apply firmware-side thresholds initially; send raw analog values so database / backend logic can classify them later.

Hardware / pin note

Stephen asked whether a second line on GPIO 0 could be sampled like DHT. The current firmware config comments warn that GPIO 0, 2, 12, and 15 are used for boot mode selection and may not be good wake-up sources, so this issue should confirm the final pin choice before implementation.

Acceptance criteria

• Firmware has a configurable analog input pin for the new sensor channel.
• The analog value is sampled on the same cadence as the existing extra sensor reads, unless a better cadence is intentionally chosen.
• Readings are included in outgoing reporting in a way the backend can store and threshold later.
• Existing DHT humidity / temperature readings continue to work.
• GPIO 0 suitability is verified or an alternate safe ADC-capable pin is selected and documented.

Source

From Stephen Peacock's punchlist:

One powerful feature is to add a second input. high importance. IR sensors for solar pumps, grease detection, handle movement etc.

Could you add a second line on GPIO 0 to sample like DHT does? This will work.

We can just take a single analog input. The threshold can be on the database side.

Priority order from Stephen: (2) second input for analog sensing that can be treated like DHT sensor.