Clarify/implement scope (currently always empty)

[EricAndrechek]

Area: ingest — confusion / FYI · found via WaveHouse-Stats dogfooding

Expected: know whether scope (in EventMessage) provides any isolation today.

Actual: it's always empty — the multi-tenant scope concept exists in the wire format but isn't populated or used (internal/api/ingest.go:150-151, with matching // TODO: impl scope in structured_query.go:121 and stream.go:54).

Impact (Stats): none now (single-tenant); flagging so nobody assumes scope isolates anything if tenants are added.

Scope: document that scope is currently inert, and/or implement population as part of multi-tenant work.

Related: per-table policy storage + per-table authorization scopes #214 (the policy side of multi-tenant; scope is the runtime-population half).

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From WAVEHOUSE-FEEDBACK.md (WaveHouse-Stats dogfooding); validated by code-read against 0f8826c on 2026-06-04.

[EricAndrechek]

Additional scope-TODO sites (from a codebase TODO sweep, 2026-06-04) — beyond the ingest/query/stream // TODO: impl scope markers already noted, scope is also unimplemented in the pipes + DLQ paths:

• internal/api/pipes.go:140 — // TODO: scope impl
• internal/api/dlq.go:51 — // TODO: do we need to break out scopes here?

Worth covering these when scope is implemented so it's consistent across ingest / query / stream / pipes / DLQ.