SLT-1103: Frontend chart: Add support for mailpit, deprecate mailhog

[k4lv15]

Motivation:
Mailhog is an email service we’re using in our development environments (local & Silta non-production) to test out if email functionalities are working properly. Mailhog mailhog/MailHog#442 (comment) and is causing issues for developers. Therefore, we need to replace it with some alternative. Our current pick is Mailpit.

Changes proposed:

• Add mailpit dependency, enable it, add default value overrides
• Update chart templates with needed changes for mailpit to work
• Keep mailhog in chart but add deprecation notices

How to test:

1. Install the updated chart and see that mailpit works (helm install frontend-release charts/frontend --set mailpit.enabled=true or test here: https://app.circleci.com/pipelines/github/wunderio/frontend-project-k8s/1544/workflows/1c038599-e79c-4987-8fd3-b3b47340c4c3). Test the following:

• validation goes through
• mailpit URL is listed in the release notes and it works
• the legacy /mailhog path redirects to /mailpit
• try sending an email from front-end, i.e., ssh into either hello or world pods run printf "EHLO localhost\r\nMAIL FROM:<test@example.com>\r\nRCPT TO:<recipient@example.com>\r\nDATA\r\nSubject: Test Email\r\n\r\nThis is a test email.\r\n.\r\nQUIT\r\n" | nc $MAILPIT_ADDRESS 25, see that the command is successful and there is a new e-mail in mailpit.
• Test that mailpit's UI works, i.e, you can mark emails as read, delete them, download etc.

2. See that updated installation checks work, i.e., try to install chart with both mailpit and mailhog enabled (i.e., run helm install frontend-release charts/frontend --set mailpit.enabled=true --set mailhog.enabled=true --dry-run), see that the installation fails.

[Copilot]

Pull Request Overview

This PR introduces support for Mailpit while deprecating Mailhog as the email testing service in non-production environments. Key changes include:

• Adding Mailpit configuration and dependency in silta.yml and Chart.yaml.
• Updating charts/frontend values and template files to redirect legacy Mailhog paths to Mailpit.
• Updating tests and CI configuration to accommodate the Mailpit integration.

Reviewed Changes

Copilot reviewed 9 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
silta/silta.yml	Added Mailpit service enabled flag.
charts/frontend/values.yaml	Added Mailpit configuration including environment variables and resources; nested key naming may be confusing.
charts/frontend/test.values.yaml	Updated test values changing Mailhog to Mailpit.
charts/frontend/templates/varnish-configmap-vcl.yaml	Introduced conditional block to bypass varnish for Mailpit paths.
charts/frontend/templates/configmap.yaml	Redirect legacy /mailhog to /mailpit with proxy configuration.
charts/frontend/templates/checks.yaml	Added failure checks for scenarios where both Mailpit and Mailhog or Mailpit in production are enabled.
charts/frontend/Chart.yaml	Added Mailpit dependency with corresponding configuration condition.
.circleci/config.yml	Updated CI to include the Mailpit helm repository.

Files not reviewed (3)

• charts/frontend/templates/NOTES.txt: Language not supported
• charts/frontend/templates/_helpers.tpl: Language not supported
• charts/frontend/values.schema.json: Language not supported

Comments suppressed due to low confidence (1)

charts/frontend/values.yaml:595

• [nitpick] The nested 'mailpit' block within the Mailpit configuration may be confusing. Consider renaming it to something more descriptive, like 'web' or 'proxy', to clarify its purpose.

mailpit: