Confluence import: app layer, worker, and state machine

[Willyfrog]

Layer 5 of 7 in the Confluence page import stack, split out of #18. The design narrative and the accepted risks live in #18 — read it first; this PR is one layer of it.

Stack order: #30 → #31 → #32 → #33 → #34 → #35 → #36

The orchestration core: admission, preflight, confirmation, execution, retry, terminalization, and cleanup. This is the layer with the most behaviour per line and the one that most deserves a senior reviewer.

What's here

import.go, import_preflight.go, import_confirm.go, import_execute.go, import_provision.go, import_retry.go, import_report.go, import_cleanup.go, plus service.go and ws_events.go wiring.

What to look for

• Nothing is written before the user agrees to it, and the agreement names a specific plan revision. A confirmation for a revision that no longer describes the source must be refused and the job requeued for re-review — check the revision comparison can't be satisfied by a stale-but-equal value.
• Bounded, paced retries. A failing job steps aside so unrelated imports proceed; one that keeps failing is failed with a report rather than retried forever. Critically: a pass that committed pages must not spend an attempt. A regression here already broke same-source serialization once (retry cooldown).
• Unified terminalization — every path out of the state machine goes through it, so "job ended without a report" should be unrepresentable. Try to find a path that isn't.
• Retained accounting — mandatory per-entity outcomes are reserved up front so an admitted job can always record what happened to every page. Check an admitted job can't exhaust the budget mid-run.
• The HA risk is accepted and stated in Confluence page import: importer, worker, execution, reports, and wizard #18: a second node would requeue this node's in-flight preflight repeatedly. V1 is single-node by design. Worth re-reading that argument here rather than re-litigating it.

Verification

go build ./..., go vet ./... clean. server/app suite green.