Organization groundwork: org-scoped connection plumbing

Preparation stack for the organization system: every database unit of work moves inside a transaction that carries the request's organization context as SET LOCAL settings (app.org_id, app.is_system_admin). This is the substrate later row-level-security enforcement keys on.

Behavior-invisible by design — no endpoint changes its responses while this stack lands; permission output is proven byte-identical with the golden-file harness at each step, and the full suite stays green throughout.

Planned as three PRs, merged in order:

1. Core plumbing — the org_scope module (scoped-connection context manager + FastAPI dependency), middleware org derivation from the active instance, permission-check path on scoped connections, and a guard making the empty-string org id (the no-context sentinel) impossible to collide with a real organization. (PR link when open.)
2. Admin surface — session/site/instance, user-management, token and backup handlers take their connection from the dependency; org-context parameter (defaulting to the user's sole org) on the endpoints that have no active instance.
3. Adversarial test skeleton — two-organization fixtures, cross-org negative tests that execute today and document current behavior (expected-failure marks until enforcement), and a build-failing canary that flags any handler resolving a database connection outside the sanctioned path, with a reviewed allowlist.

Working rule stated in code and docs: one unit of work = one scoped-connection acquisition; a logically atomic read-modify-write never splits across two.

Streaming surfaces (SSE/console/monitoring) and the remaining DB-backed routers follow in two further batches, after which the canary's burn-down list is empty. Behavior-changing hardening items are tracked as separate issues.

Begin with the planned org_scope module and FastAPI dependency, then trace middleware organization derivation and the permission-check path on scoped connections. Use the golden-file harness and full suite as the first checks; done requires the ordered plumbing, admin handlers, adversarial fixtures, and canary described in the plan.