feat: notification content options Phase 2 — Jinja2 title/body template strings

[gregoryfoster]

Summary

Phase 2 of #88. Add title_template and body_template Jinja2 string fields to ContentOptions, enabling fully custom notification titles and bodies per config.

Design context

docs/plans/2026-04-14-notification-content-options-design.md — Phase 2 section:

ContentOptions gains title_template / body_template Jinja2 string fields. No schema migration needed — JSONB column already in place.

Scope

• Add title_template: str | None and body_template: str | None to ContentOptions
• Template context: all fields currently in WatchEvent + metadata keys (diff_lines, effective_domain, check_interval, etc.)
• build_body() renders body_template via Jinja2 when set; falls back to Phase 1 additive composition otherwise
• dispatch_event title override: render title_template when set (currently title is unchanged)
• Dashboard: textarea inputs for template strings in the Content Options section (collapsed by default)
• Template preview: render against a synthetic event (deferred if needed — can ship templates without live preview)

Out of scope

• AI-powered summaries (separate future item)
• Template variable autocomplete in the UI

[gregoryfoster]

Implemented in commits 44ea3ce–be7814a.

• Added title_template: str | None and body_template: str | None to ContentOptions
• build_body() short-circuits to Jinja2 render when body_template is set; additive sections suppressed
• Title rendered via render_template() in notify.py before dispatch_event() call; falls back to event.title when unset
• build_template_context() provides context: all WatchEvent fields + flattened metadata keys
• Dashboard: textarea inputs for both templates in the Content Options accordion (collapsed by default)
• Error handling: malformed Jinja2 falls back to raw template string, never crashes dispatch
• Template preview deferred as noted in issue