A4: Alert notifications — email (SMTP) [deferred, follow-up to A3]

[LarsLaskowski]

Epic: Alerting & Persistence

Deferred: webhook delivery (A3) ships first. Pick this up once the notifier interface from A3 exists.

Why

A common ask for headless home servers that have no chat platform: send alert notifications by email.

Scope

An optional SMTP sink using net/smtp (standard library) with STARTTLS. Optional digest mode to batch multiple alerts into one message.

Reuse & touch points

• Extends the notifier interface introduced in A3 (internal/alert/notify.go).
• internal/config/config.go — new alerts.email: block (host/port/from/to/auth).

API & config impact

Config only. No new runtime dependency (std-lib net/smtp).

Acceptance criteria

• Against a stub SMTP server, a fired alert produces a well-formed message.
• A misconfigured mailer logs an error but never blocks metric collection.