feat: per-event title_template and body_template in content options

[gregoryfoster]

Summary

The per-event overrides accordion (#90) allows customising additive content options (diff, domain, significance, etc.) per event type. However, title_template and body_template Jinja2 strings are only available at the default level — they cannot be set per event type through the UI or form parser.

A user who wants a custom Jinja2 title for change_detected events but the default title for watch_error events has no path to do this.

Scope

• Extend _parse_content_config_from_form() to read content_config__override__{et}__title_template and content_config__override__{et}__body_template textarea inputs
• Add textarea inputs to each per-event accordion row in notification_content_options.html
• Add tests for round-trip

Notes

• No backend changes needed — ContentOptions.title_template / body_template already exist and resolve_options() already applies per-event overrides
• render_template() and build_template_context() are already called with the per-event resolved options in notify.py
• Pure UI + form-parsing work, same pattern as feat: per-event-type content options UI accordion #90