feat: Fan failure detection and alerting

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Feature Description

Detect when fans fail or stall and provide alerts.

Detection Methods

1. RPM Monitoring

• Fan should be running but RPM = 0
• Stalled fan (RPM below threshold despite PWM > 0)
• Disconnected fan (no tach signal)

2. Threshold-Based

• Minimum RPM per fan configurable
• Timeout for detection (avoid false positives)
• Distinguish between stopped vs. failed

Alert Mechanisms

USB Notifications

• Status Report (0x10) includes error flags
• Error codes per fan
• Host software can show notifications

Visual Alerts

• Status LED blinks pattern
• RGB LEDs flash red
• Display shows error (if OLED present)

Audible Alert (Optional)

• Buzzer on GPIO
• Beep pattern for critical failures

Actions on Failure

Safety Actions:

• Other fans → 100% (compensate)
• Log event in flash
• Keep trying to restart failed fan
• Report to host

User Notification:

• Desktop notification (Linux)
• System tray icon change
• Log to syslog

Configuration

pico-fan-ctl config alert --enable
pico-fan-ctl config alert --min-rpm 300
pico-fan-ctl config alert --timeout 10s
pico-fan-ctl config alert --action compensate

Priority

High - Safety critical feature