Stage 4: Notification core — Recipients, templates, email sending

[bakhterets]

Implement the core notification logic including recipient resolution, template rendering, and SMTP sending.

Files: internal/notification/notification.go, internal/notification/resolver.go, internal/notification/renderer.go, internal/notification/smtp.go, internal/notification/templates/

Tasks:

• Define event kinds as typed constants: pending_review, reviewed, status_changed.
• Recipient resolver: review audience (SMOD fixed address + operator + admin lists) + contact_email, normalize + dedup.
• Build dedup_key as change_id : kind : recipient and store in JSONB.
• Build and serialize payload snapshot (title, dates, old/new status, actor, incident link).
• Change summary builder (title, dates, old/new status).
• Email templates in internal/notification/templates/ (subject.txt, body.html) using text/template.
• Email renderer (subject, body, deep link from payload).
• SMTP sender using github.com/wneessen/go-mail with timeout + TLS.
• Promote google/uuid to a direct dependency.
• Verify: unit tests for kind mapping, recipient rules, dedup key, summary, payload serialization, and rendering.

Deliverable: Core notification system with templating, recipient resolution, and SMTP integration.