feat(integrations): add Mailgun docs and example

[djbriane]

Summary

Adds Mailgun as a documented integration: docs, runnable example, and homepage integration card.

Changes

• Docs: apps/docs/integrations/mailgun.mdx – install, render + send with Mailgun, link to example
• Example: examples/mailgun/ – minimal send script using mailgun.js, render() from @react-email/components, and FormData (aligns with other provider examples)
• Discovery: Mailgun added to docs nav (docs.json), integrations snippets, README integrations list, and homepage “Integrate with any service” section (integration.tsx)
• Skill: Mailgun block in skills/react-email/references/SENDING.md under “Send with Other Providers”

Running the example

From examples/mailgun: install deps, build (pnpm run build or equivalent), then run with MAILGUN_API_KEY and MAILGUN_DOMAIN set. Not in pnpm workspace by design; run from the example directory.

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Summary by cubic

Add Mailgun integration docs and a runnable example, plus a homepage integration card. This helps users render emails with React Email and send them via Mailgun.

• New Features
  • Docs: apps/docs/integrations/mailgun.mdx with install, render, and send steps; links to the example.
  • Example: examples/mailgun/ using mailgun.js, form-data, and render() from @react-email/components.
  • Discovery: added to docs nav, integrations snippets, README list, and homepage integrations section with logo.
  • Guides: added a Mailgun section to “Send with Other Providers” in skills/react-email/references/SENDING.md.

^{Written for commit 6d23ecf. Summary will update on new commits.}

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Latest commit: 6d23ecf

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[cubic-dev-ai]

4 issues found across 10 files

Confidence score: 3/5

• There is a concrete user-impacting risk: new client.messages.create usage in examples/mailgun/src/index.tsx does not surface/guide on API-key permission failures, which can lead to confusing send failures in production.
• apps/docs/integrations/mailgun.mdx and skills/react-email/references/SENDING.md both miss an explicit MAILGUN_API_KEY send-scope reminder for this SDK method, increasing the chance of misconfigured deployments.
• Score is 3 because multiple medium-high severity findings (7/10, high confidence) point to likely integration/regression friction rather than a guaranteed runtime break.
• Pay close attention to examples/mailgun/src/index.tsx, apps/docs/integrations/mailgun.mdx, skills/react-email/references/SENDING.md - permission-scope checks/reminders and EU-region Mailgun configuration need to be explicit to avoid avoidable send failures.

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="apps/docs/integrations/mailgun.mdx">

<violation number="1" location="apps/docs/integrations/mailgun.mdx:60">
P1: Custom agent: **API Key Permission Check SDK Methods**

Add an explicit permission-scope reminder for `MAILGUN_API_KEY` when using `client.messages.create`, so production keys are verified to have send access before deployment.</violation>
</file>

<file name="skills/react-email/references/SENDING.md">

<violation number="1" location="skills/react-email/references/SENDING.md:115">
P1: Custom agent: **API Key Permission Check SDK Methods**

The new Mailgun `messages.create` SDK usage needs an explicit API-key permission check/reminder. Add a guard or note to verify production `MAILGUN_API_KEY` can send messages for the domain before invoking this method.</violation>
</file>

<file name="examples/mailgun/src/index.tsx">

<violation number="1" location="examples/mailgun/src/index.tsx:7">
P2: Mailgun client config is not region-aware; EU accounts require an explicit EU API URL, so this example can fail for EU domains.</violation>

<violation number="2" location="examples/mailgun/src/index.tsx:14">
P1: Custom agent: **API Key Permission Check SDK Methods**

`client.messages.create` introduces a new Mailgun SDK operation, but the new code does not handle/flag permission failures. Per the API Key Permission Check SDK Methods rule, add an explicit 401/403 guard that tells operators to verify the production `MAILGUN_API_KEY` has send permissions for the configured domain.</violation>
</file>

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[djbriane]

In regards to the @cubic-dev-ai feedback, I am recommending we do not implement those changes. I'd be open to adding the region note if maintainers suggest it, but overall the current state feels aligned with how the other provider examples work.