Model distro addresses as routes, not Graph mailboxes

Reusable email-onboarding design note: Microsoft Graph Outlook subscriptions target user/shared mailboxes with mail folders; a traditional distribution group has no Inbox and should be modeled as an address route delivered into an already monitored sink mailbox. Keep mailbox policy/subscription lifecycle separate from route-level controls. If per-distro reply toggles are required, persist the matched route on each message and enforce global automation first, then route enabled/epoch, again immediately before send. Provenance: Microsoft Outlook mail API overview, Exchange Online collaboration docs, Graph subscription docs, and a read-only application architecture audit. Sources: https://learn.microsoft.com/en-us/graph/api/resources/mail-api-overview?view=graph-rest-1.0 ; https://learn.microsoft.com/en-us/exchange/collaboration-exo/collaboration-exo ; https://learn.microsoft.com/en-us/graph/api/subscription-post-subscriptions?view=graph-rest-1.0