feat: reserved outbox + unique sequence stream for delivery webhooks

[mkostromin-sigma]

Parent

Parent: #1606

Blocked by

• Unblock/merge of fix: media buy delivery correctness — scheduler serving statuses + webhook-only fields #1575 (and L0 leaf if still open after merge)

Scope

• Alembic migration: UNIQUE (tenant_id, media_buy_id, task_type, sequence_number) on webhook_delivery_log
• Status reserved (plus existing success/failed/retrying)
• DeliveryRepository.reserve_next_sequence(...) + mark_delivery_result(...)
• Repository integration tests only; no scheduler behavior change yet

Spec grounding

UC-004 monotonic sequence_number — docs/test-obligations/UC-004-deliver-media-buy-metrics.md; AdCP 3.1.0-beta.3 reporting webhook. Lifecycle-final ungraded.