feat(queue): implement real email transport in the email processor

[Wittig18]

Summary

• Closes Implement a real email transport in the email queue processor #362
• Replaces the simulated simulateEmailSend timeout in the email queue processor with a pluggable EmailTransport selected from validated env config (EMAIL_PROVIDER, SMTP/SES/SendGrid settings).
• Provider failures now throw so BullMQ marks the job failed and retries instead of returning { success: true } with no delivery.
• Adds strict recipient validation and CR/LF header-injection guards; logs use the structured logger without full recipient addresses or bodies at info level.

Changes

• src/queue/processors/email.transport.ts — EmailTransport interface, ConsoleEmailTransport, SendGridEmailTransport (SendGrid v3 REST via axios), SmtpEmailTransport, SesEmailTransport (AWS SigV4), setEmailTransportOverride() for tests.
• src/queue/processors/email-processor.ts — Validates payload, dispatches via resolveEmailTransport(), preserves { success, message, data: { emailId } } return shape.
• src/config/env.schema.ts — Email transport env vars with Zod validation.
• src/queue/processors/email-processor.test.ts — Mock transport delivery, provider failure propagation, header-injection rejection.
• docs/email-notifications.md — Queue processor transport selection and config.

Config

Variable	Purpose
EMAIL_PROVIDER	console (default), smtp, ses, sendgrid
EMAIL_SEND_TIMEOUT_MS	Provider call timeout (default 10000)
SMTP_*	SMTP host/port/from/credentials
AWS_*	SES credentials and region
SENDGRID_API_KEY	SendGrid API key

Test plan

• npm run lint
• npm test -- src/queue/processors/email-processor.test.ts
• Enqueue an email-notification job with EMAIL_PROVIDER=console and confirm success
• Inject a failing transport (or misconfigure provider) and confirm the job fails/retries
• Confirm invalid recipient / header-injection payloads reject before dispatch

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[Wittig18]

@mikewheeleer please merge