feat: content option — include last changed date (include_last_changed_at)

[gregoryfoster]

Summary

Add include_last_changed_at: bool = False to ContentOptions. Surfaces Watch.last_changed_at as "Last changed: 2026-04-09" — gives recipients frequency context that a bare change notification lacks.

Context

Follows from #88 (notification content options Phase 1). The design doc listed last_changed_at under temporal context but the implementation only shipped check_interval. This adds the missing half. No schema migration needed.

Scope

Worker (src/workers/tasks.py):

• Add last_changed_at to _watch_base_metadata(watch):

  if watch.last_changed_at:
      meta["last_changed_at"] = watch.last_changed_at.strftime("%Y-%m-%d")

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

• Add include_last_changed_at: bool = False to ContentOptions
• Extend _build_temporal_section(metadata, options) (or add a separate section) to render "Last changed: {date}" when last_changed_at is present and include_last_changed_at is True

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

• Add checkbox alongside existing "Temporal context": "Last changed date"
• Consider grouping with include_temporal_context under a single "Temporal" label

Notes

• last_changed_at is None until the first change is detected — render silently omitted when absent
• ISO date format (YYYY-MM-DD) per project conventions; no time component needed (date is sufficient for frequency context)
• Could be folded into the existing include_temporal_context checkbox rather than a separate toggle — design decision at implementation time

[gregoryfoster]

Implemented in commits adf5296, deac35a, 0b7cff1.

• Added last_changed_at: DateTime column to watches table (migration fa835a9e496d) — issue assumed the field existed; it didn't
• _watch_base_metadata() now injects last_changed_at as ISO date string (YYYY-MM-DD) when set
• include_last_changed_at: bool = False added to ContentOptions; _build_last_changed_section() added to content builder
• DB trigger (migration ecf003d279f1): AFTER INSERT ON changes stamps watches.last_changed_at = NEW.detected_at automatically — no application-level write needed, no code path can miss it
• Dashboard checkbox added; tests in TestBuildLastChangedSection + two integration tests verifying trigger fires/does-not-fire
• Note: watches.updated_at is not touched by the trigger (bypasses SQLAlchemy ORM onupdate); this is intentional — updated_at tracks config edits, not content changes