Plan 03: Simplify SourceHealthMonitor and SourceHealthReset

Goal

Rewrite the determine_status decision tree to produce only 4 health values and decouple auto-pause from health diagnosis. Update SourceHealthReset to use the new default status.

Tasks

Task 1: Rewrite determine_status in SourceHealthMonitor

Files: lib/source_monitor/health/source_health_monitor.rb

Replace the current determine_status method with the simplified decision tree:

1. rate >= healthy_threshold -> "working"
2. rate < auto_pause_threshold -> "failing"
3. consecutive_failures(logs) >= 3 -> "declining"
4. improving_streak?(logs) -> "improving"
5. Fallback (between thresholds, no streak) -> "declining"

Remove the auto_paused_active? check from determine_status -- auto-pause is an operational concern handled separately by the auto-pause logic already in call.

Remove the warning_threshold method entirely. Update healthy_threshold to no longer reference warning_threshold.

Update apply_status fallback from "healthy" to "working".

Task 2: Update SourceHealthReset default status

Files: lib/source_monitor/health/source_health_reset.rb

Change health_status: "healthy" to health_status: "working" in reset_attributes.

Task 3: Update ImportSourceHealthCheck status values

Files: lib/source_monitor/health/import_source_health_check.rb

Change "healthy" to "working" and "unhealthy" to "failing" in the Result structs. This aligns the import wizard health check with the new status vocabulary.