Phase 2 of the dashboard route consolidation. The backend now double-registers
every bare top-level route (approve/deny/kill/restart/start/rebuild/destroy/
update-all/answer-question/cancel-question/purge-tombstone/matrix-account-login/
cancel-reminder/retry-reminder/request-spawn/op-send/meta-update/dashboard-stream/
dashboard-history) at an additional /api/<same> path. Switch every dashboard-pkg
fetch / EventSource / form action to the /api/ form so "backend = /api/*" holds
on the frontend side too.
Bare paths still answer, so this is independently deployable; once it ships the
backend can drop the bare registrations (phase 3). webhook/knowledge stays a
distinct webhook prefix (forge-driven, no SPA caller). app.js's /rebuild,/start
are the agent harness UI (different server) and are untouched.
Part of #1846 (phase 2).
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