v0.20.0

Added

• Durable step persistence — a model-backed, owner-scoped store for
  pydantic-ai-harness's StepPersistence capability. Pass
  AGUIServer(step_store=DefaultStepStore) (the constructor is the
  request -> StepStore factory — the harness protocol carries no request, so
  the store binds one and is built per run) and every run records an append-only
  event log, a (run_id, tool_call_id) tool-effect ledger, and a continuable
  snapshot at each provider-valid boundary, keyed on the AG-UI run_id. Four new
  models under django_ag_ui.contrib.store (StoredRun / StoredStepEvent /
  StoredSnapshot / StoredToolEffect, migration 0003) back
  DefaultStepStore, which structurally satisfies the harness StepStore
  protocol. Every row filters by the resolved owner, so a run_id from one user
  can't read another's runs; an anonymous request without ALLOW_ANONYMOUS
  degrades to no-op rather than aborting the run. Requires the [harness] extra.
  A custom backend is any request -> StepStore callable. See
  Durable step persistence.
• Resume / fork endpoints — configuring a step_store also mounts owner-scoped
  resume/<run_id>/ and fork/<run_id>/ endpoints. Both seed a new run from a
  prior run's last continuable snapshot: the server loads it (a run_id from
  another owner is a clean 404), injects it as the run's message_history
  (AgentSession gained the seam; run_stream_native composes it ahead of the
  client's new turn), and records the new run with parent_run_id pointing back
  at the source — so the parent is never mutated. resume and fork are two
  names for one mechanism (the harness's continue_run / fork_run are
  data-identical). The web-component checkpoint UI rides a downstream release.