docs: add account metrics endpoint

[dielduarte]

Adds GET /metrics — account-level email metrics for a date range — under a new Metrics tag.

Shape

Two blocks:

• overview — volume/delivery counts (sent = delivered + bounced + failed), with optional per-period timeseries and per-domain domain_breakdown.
• engagement — unique opens/clicks as { unique, delivered, rate } (delivered = tracking-eligible denominator; rate is a percentage capped at 100), with the same optional breakdowns.

Params

• start_date / end_date (required, ISO 8601)
• timezone (default UTC), domain_id (uuid)
• event_type — repeatable; projects the overview block to the listed events (does not affect engagement)
• include — repeatable; adds timeseries and/or domain_breakdown to both blocks (default: totals only)

Source of truth is resend.yaml only (resend.json is auto-generated on merge to main). New tag: Metrics — the account endpoint has no parent entity, unlike the broadcast metrics endpoint which lives under Broadcasts.

Implements the response contract from resend-monorepo #6302.

[dielduarte]

Superseded by a 2-PR stack: overview (#77) and engagement (stacked).