test(notifications): cover default leaving soon email template rendering

[rfsbraz]

• The reported bug (missing default email template) turned out to be invalid: DEFAULT_TEMPLATE_PATH resolves to app/templates/leaving_soon.html, which exists on main, ships in the Docker image via COPY ./app /app/app, and renders correctly with the provider's context.
• The real gap was test coverage: nothing guarded against the template going missing or breaking, which would make every email silently fall back to the built-in HTML with only a warning log.
• Adds regression tests asserting the default template exists and renders through _render_leaving_soon_template with full and minimal context (items, deletion date, Plex/Seerr links, totals).

[github-actions]

🐳 To test this PR, build and run directly from the branch:

Using docker compose:

services:
  deleterr:
    build: https://github.com/rfsbraz/deleterr.git#refs/pull/281/head
    volumes:
      - ./config:/config
    environment:
      LOG_LEVEL: DEBUG

Using docker build:

docker build https://github.com/rfsbraz/deleterr.git#refs/pull/281/head -t deleterr:pr-281
docker run --rm -v ./config:/config -e LOG_LEVEL=DEBUG deleterr:pr-281

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud