feat: content option — include dashboard link to change (include_change_dashboard_url)

[gregoryfoster]

Summary

Add include_change_dashboard_url: bool = False to ContentOptions. When enabled, appends a direct link to the change diff view in the dashboard — lets recipients click through without hunting for the watch.

Context

Follows from #88 (notification content options Phase 1). No schema migration needed — JSONB column already in place.

Scope

Pipeline (src/workers/pipeline.py):

• Add "change_id": str(change.id) to change_metadata dict (line ~298, after session.flush())

Worker (src/workers/tasks.py):

• No changes needed — change_id flows through change_metadata into event metadata automatically

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

• Add include_change_dashboard_url: bool = False to ContentOptions
• Add _build_change_url_section(watch_id, metadata) — constructs {APP_URL}/watches/{watch_id}/changes/{change_id}
• Only renders when change_id is present in metadata (i.e., change_detected events only)
• APP_URL sourced from env var or config (already used in dispatcher.py via AppriseAsset.app_url)

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

• Add checkbox: "Dashboard link (change URL)"

Notes

• Only meaningful for change_detected events; silently omitted for others (no change_id in metadata)
• app_url is already defined as "https://watcher.exe.xyz" in dispatcher.py — extract to a shared constant or config value rather than duplicating

[gregoryfoster]

Implemented in commit 8175ffa.

• Added "change_id": str(change.id) to change_metadata in _run_check_pipeline() after Change INSERT
• include_change_dashboard_url: bool = False added to ContentOptions
• _build_change_url_section(watch_id, metadata) constructs {APP_URL}/watches/{id}/changes/{change_id}
• APP_URL extracted to src/core/notifications/constants.py to break circular import between content.py and dispatcher.py
• Section silently omits for non-change events (no change_id in metadata)
• Dashboard checkbox added; tests in TestBuildChangeUrlSection