Migrate HVAC filter change capture to NFC stickers

[CCOSTAN]

Goal

Move HVAC filter change logging to NFC stickers on the HVAC units so capture is simple, consistent, and timestamped automatically.

Scope

• Add one NFC sticker on the upstairs HVAC unit.
• Add one NFC sticker on the downstairs HVAC unit.
• Each sticker should log a filter change for its specific unit with the current date/time.
• Keep Joanna focused on retrieval and stats, not filter-change capture, once the NFC path is in place.

Acceptance criteria

• Scanning the upstairs sticker logs an upstairs HVAC filter change event.
• Scanning the downstairs sticker logs a downstairs HVAC filter change event.
• Each event records the unit, date/time, and filter-change count/state needed for calculations.
• Home Assistant exposes enough state/history for days since last change and historical cadence per unit.
• Joanna can answer retrieval questions such as when the upstairs or downstairs filter was last changed.
• Any old manual/voice capture fast paths for HVAC filter changes are removed or narrowed after NFC capture is working.

Notes

This should mirror the water softener NFC migration pattern, but HVAC needs two physical stickers because upstairs and downstairs are separate maintenance targets.