Construct backend interfaces from a shared server

[floitsch]

Depends on #1253.

Why

The CLI should depend on the roles it needs, while transport, authentication,
scope, and connection lifetime can still be shared. A combined backend object
would preserve the coupling we are trying to remove and would make it harder
to assign different implementations to different roles later.

What

• Introduce a Server connection abstraction that owns transport,
  authentication, scope, and lifetime.
• Define independent role interfaces:
  • UpdateBroker publishes desired update state and handles provisioning.
  • ArtifactStore stores firmware and application images.
  • PodStore stores pod metadata, manifests, and parts.
• Model reported state and events as separate optional broker capabilities:
  BrokerStateReader and BrokerEventReader.
• Do not introduce a remote FleetStore: the fleet is currently the local
  filesystem/YAML model, and belongs behind a different future boundary.
• Construct HTTP or Supabase implementations for each role from the configured
  Server; there is no CombinedBackend adapter.
• Remove the generic tenancy setting. When the same configured server is
  assigned to both Artemis and broker roles, construct the combined
  UpdateBroker implementation that also registers the device with Artemis.
• Keep standalone and combined provisioning behavior in separate HTTP and
  Supabase update-broker implementations.
• Keep the current wire protocol and /functions/v1/b endpoint unchanged.

This matches the intended configuration model: construct a server and assign
its implementations to selected roles. Later configurations can mix providers,
and the Supabase v1 edge functions can split by interface without changing the
CLI call sites again.

Verification

• Analyzed the affected Toit sources and tests.
• Ran broker-test.toit with standalone and combined HTTP brokers.
• Ran pod-registry-test.toit with standalone and combined HTTP brokers.
• Supabase verification runs in CI; the combined implementation now preserves
  the two-argument new_provisioned RPC payload.