feat: content option — include change significance score (include_significance)

[gregoryfoster]

Summary

Add include_significance: bool = False to ContentOptions. Surfaces the already-computed significance score as "Significance: 73%" in the notification body — lets recipients triage at a glance without reading the diff.

Context

Follows from #88 (notification content options Phase 1). significance is already computed in the pipeline (_compute_significance) but is not currently passed into change_metadata or event metadata. No schema migration needed.

Scope

Pipeline (src/workers/pipeline.py):

• Add "significance": significance to change_metadata dict (line ~298, alongside added/removed/modified)

Content builder (src/core/notifications/content.py):

• Add include_significance: bool = False to ContentOptions
• Add _build_significance_section(metadata) — renders "Significance: {pct}%" where pct = int(metadata["significance"] * 100)
• Only renders when significance key is present in metadata

Dashboard (src/dashboard/templates/partials/notification_content_options.html):

• Add checkbox: "Change significance score"

Notes

• Only meaningful for change_detected events; silently omitted for others
• significance is a float 0–1; display as integer percentage (e.g. 0.73 → "73%")
• _compute_significance clamps to [0, 1] so no bounds check needed at render time

[gregoryfoster]

Implemented in commit fd63e8a.

• Added "significance": significance to change_metadata in _run_check_pipeline()
• include_significance: bool = False added to ContentOptions
• _build_significance_section(metadata) renders "Significance: N%" (float → integer percentage)
• Handles 0.0 correctly (renders "Significance: 0%", not suppressed)
• Section silently omits for events without a significance key in metadata
• Dashboard checkbox added; tests in TestBuildSignificanceSection