PubSub broadcasting for workflow step lifecycle events.

Workers broadcast on two topic levels so LiveViews can subscribe at whatever granularity they need:

• "workflow:all" — every event from every workflow (for the index view)
• "workflow:{workflow_id}" — events for one specific workflow (for the detail view)

Event shapes

# Step changed state
{:workflow_step_updated, %{
  workflow_id: "uuid",
  workflow_label: "my-etl",
  step_name: "fetch_token",
  worker: "MyApp.Workers.FetchToken",
  state: "completed",         # see the state list below
  job_id: 123,
  attempt: 1,
  has_result: true,
  error: nil,                 # or error string on failure
  detail: nil,                # or a state-specific map (see "awaiting")
  timestamp: ~U[...]
}}

state is one of "executing", "snoozed", "awaiting", "expanded", "completed", "retryable", "discarded", or "cancelled". Consumers that match on it should keep a catch-all clause; new states are added as the engine grows ways to wait.

"expanded" is not a resting state: it reports that a dynamic fan-out step just created detail.count new steps (Baton.Expansion) and is about to park until they finish. It is the one event that means the graph itself changed shape.

"awaiting" means the step is parked on work happening somewhere else — a provider batch that will take hours — as opposed to "snoozed", which means it is waiting for a resource of its own: detail carries %{reason: r, seconds: n} where reason is "deps" (upstream dependency pending), "rate_budget" (host rate limiter starved), "provider_limit" (provider 429/529), "batch_slot" (host batch-submission slot unavailable), or "step" (the step snoozed itself), and seconds is the announced wait — what the step asked Oban for, an upper bound on the actual wait. "awaiting"'s detail map is %{batch_id: "msgbatch_...", seconds: n} and repeats on every poll, so a UI can show what is being waited on rather than a step that appears to flicker.

# Whole workflow reached terminal state
{:workflow_finished, %{
  workflow_id: "uuid",
  workflow_label: "my-etl",
  outcome: :completed,        # or :failed
  failed_steps: ["step_b"],
  timestamp: ~U[...]
}}