Add Mailgun email provider (WASM module)

[jeroenrinzema]

Goal

Add a new mailgun provider so Lunogram can send email through Mailgun, based on the same provider-level scope and expectations as #216.

Scope

• Add Mailgun support for the email channel (providers.ChannelEmail)
• Define config requirements for credentials (at minimum Mailgun API key and sending domain)
• Implement send behavior mapping Lunogram email payload -> Mailgun API request
• Validate required configuration before send
• (If feasible) add webhook handling for delivery events mapped to canonical provider webhook events

Provider expectations

• Follow the current provider architecture and contracts used by Lunogram
• Include provider metadata (id, title, icon/color, website, rate limit)
• Keep error handling explicit and classify permanent vs transient failures where possible

Acceptance criteria

• Provider appears in provider metadata/list endpoints with module id mailgun
• Creating a provider with valid config succeeds
• Email send works end-to-end for HTML/text payloads
• Clear permanent/transient error classification is implemented
• Tests added/updated for core send + validation behavior

Out of scope

• SMS or push support
• Advanced Mailgun features (templates, suppressions, routing rules, etc.) in first version