Add SendGrid email provider (WASM module)

[jeroenrinzema]

Goal

Add a new sendgrid provider so Lunogram can send email through SendGrid, following the same provider model as existing modules (e.g. resend).

Scope

• New provider module under modules/providers/sendgrid/
• Email channel support only (providers.ChannelEmail)
• Config schema for required credentials (at minimum SendGrid API key)
• Send implementation mapping Lunogram email payload -> SendGrid API request
• Provider validation for required config
• (If feasible) webhook handling for delivery events mapped to canonical provider webhook events

Implementation notes

• Mirror structure and conventions from modules/providers/resend/
• Expose standard exports: manifest, send, validate, and optionally webhook, init, destroy if webhook lifecycle is implemented
• Include provider metadata (id, title, icon/color, website, rate limit)

Acceptance criteria

• Provider appears in provider metadata/list endpoints with module id sendgrid
• 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 SendGrid features (templates, marketing campaigns, etc.) in first version