Deliver scoped metrics reports from metrics rules

Goal

On the weekly metrics job, send scoped metrics reports per rule in backend/data/notifications/metrics/. Keep the global weekly community mail unchanged.

Depends on: #2114

(Can be implemented in parallel with subscription new-issues delivery; does not depend on that issue.)

Today vs after

Community delivery (unchanged)

Default weekly send: To kernelci@lists.linux.dev, Cc kernelci-results@groups.io, Saturday cron, existing cache warm.

Subscription delivery

For each rule in metrics/, filter get_metrics_data() by rule trees, hardware, origins; render metrics_report.txt.j2 for period_days; send to rule recipients. One rule spanning multiple trees → one email.

Note: Tree scoping requires SQL joins (incidents/builds/checkouts); extend cache warm for configured scopes.

Acceptance criteria

• Default config → single global weekly metrics mail unchanged
• At least one metrics rule sends scoped report to its recipients only
• Query/cache impact documented; warm covers subscribed scopes
• docs/notifications.md updated

Start in backend/data/notifications/metrics/ and inspect the metrics rules and metrics_report.txt.j2 flow, then review docs/notifications.md and the existing weekly job behavior. Verify that the default global mail is unchanged, scoped reports reach only each rule's recipients, configured scopes are covered by cache warm-up, and the query/cache impact is documented.