add server.alerts: Alertmanager with email notifications

Turns the Alertmanager wiring sketched out in conversation into an option, and
ships the alerting rules rather than leaving them as something to copy out of a
chat log.

  services.telemetry.server.alerts = {
    enable = true;
    to = [ "you@example.com" ];
    from = "alerts@example.com";
    smarthost = "smtp.fastmail.com:587";
    username = "alerts@example.com";
    passwordFile = config.sops.secrets.smtp-password.path;
  };

passwordFile becomes smtp_auth_password_file, read at runtime, so the password
never enters the store. `settings` is merged over the generated configuration
for inhibit rules and severity routing. Alerts group by alertname and host, so
nine failing drives on one machine arrive as one mail.

rules/alerts.yml carries twelve alerts: drive failing, degraded, failed
self-test, overheating; pool not ONLINE, unrepairable data errors, over 85%
full, scrub or snapshots overdue; exporter down, collector stale, textfile parse
error. Most are one-liners because rules/smartctl.yml has already reduced drive
health to a single smartctl:health_code. ExporterDown earns its place by
covering the case where a machine vanishes and every other alert goes quiet.

Alertmanager is defined inside the existing services.prometheus block: Nix will
not let one attrset both define services.prometheus wholesale and add a path
beneath it. Same shape as the grafana settings fix in 89f2c32.

Verified by building both ways. Enabled: two promtool-checked rule files, an
amtool-checked Alertmanager config with the password file, and Prometheus
pointing at 127.0.0.1:9093. Disabled: no alertmanager unit, one rule file, no
alerting stanza. The README's rule-override and alerts.settings snippets were
also built rather than written from memory.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JKczN1f66kmLxNBLX37voQ

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