Add EmailPlugin for email notifications (issue #89, Phase 3)

[stalep]

Implements email notification channel using Quarkus ReactiveMailer, following the same patterns as Horreum's EmailPlugin.

Features:

• ReactiveMailer with configurable timeout (h5m.mail.timeout, default 15s)
• Sends both HTML and plain text bodies (multipart)
• Configurable subject prefix (h5m.mail.subject.prefix, default [h5m])
• Custom subject via config: {"subject": "ALERT: {folderName}"}
• Custom body via template with {folderName}, {nodeName}, {nodeType}, {changeCount} placeholders
• Multiple recipients: {"to": "alice@example.com,bob@example.com"}
• Format-aware change details: FixedThreshold shows value/bound/direction, RelativeDifference shows ratio/previous/last
• HTML body with table of changes including fingerprint and details
• Mock mode enabled by default for dev/test; configure SMTP via MAILER_HOST, MAILER_PORT, MAILER_FROM environment variables

Dependencies:

• Added quarkus-mailer

Tests (14):

• Validation: valid config, multiple recipients, null, empty, missing to field, invalid email format
• Send: delivery, change details in body, multiple recipients, custom subject with prefix, custom template, fixed threshold formatting, relative difference formatting
• Method identity check