Design doc: queue/database ownership, queue tiering, and cross-queue message flow

[behinddwalls]

Context

Raised as a side note during review of #205 (request-log gateway consumer).

We should document the system's queue and database ownership model so the topology is explicit and reviewable.

Scope

• Ownership of queues and databases (which service owns/writes/reads what).
• Tiering of queues (start / cancel / log topics and any others, and how they relate).
• Flow of messages across queues (publish → consume → persist paths), including the request-log ownership invariant (gateway is the sole owner — writer and reader — of the request log; orchestrator only publishes log entries).

Notes

Follow-up only; not required for #205.

Co-Authored-By: Oz oz-agent@warp.dev