Update adaptors on the fly

Hypothesis

If new adaptors, versions, icons, and credential schemas appear in Lightning
without an operator restart, then services and sales teams can use them as
soon as they're released to NPM — and clustered deployments stop showing
stale or inconsistent versions to users.

Problem observations

• A new adaptor (or new version of an existing one) requires a Lightning
  restart to appear in the UI. Sales and services are blocked on releases.
• Even after restart, adaptor icons sometimes don't appear on the workflow
  diagram — they rely on a separate build step.
• Credential schemas are baked in at build time. A new adaptor schema isn't
  available until the next deploy.
• In clustered deployments, nodes can disagree about which adaptor versions
  exist, causing the version dropdown to silently downgrade a job's adaptor.
• The adaptor list, icon set, and credential schemas are each loaded by a
  different mechanism, so any fix has to address three subsystems at once.

In scope

• Adaptor registry refreshes automatically on a schedule, without restart.
• New adaptors and new versions appear across all nodes in a cluster
  consistently.
• Credential schemas are fetched from NPM at runtime and kept current.
• Adaptor icons are fetched and served at runtime; pre-baked icons still
  work in airgapped/offline deployments.
• Superusers can trigger a manual refresh from a Settings → Maintenance page
  when they don't want to wait for the schedule.
• The LOCAL_ADAPTORS mode (local filesystem source) continues to work for
  development and airgapped deployments, with no upstream NPM calls.

Out of scope

• Defining credential schemas that aren't linked to a specific adaptor
  (separate concern — see notes on #325 and #220 below).
• Per-project adaptor version pinning.
• Content-addressed icon URLs / long-lived browser caching of icons.

Sub-issues

Notes on what each contributes and what (if anything) is left for later:

• #2209 — periodic background refresh of the adaptor list, no restart
  needed. Fully addressed.
• #3114 — new adaptors' icons and schemas appear without a release.
  Both the icons-without-restart and schemas-without-restart parts are
  addressed.
• #1996 — cluster nodes disagree about adaptor versions. Addressed via
  a shared source-of-truth across nodes.
• #325 (2022) — pull credential schemas from NPM. The runtime-fetch
  part is addressed; the question of where to define non-adaptor-linked
  schemas is not.
• #220 (2022) — credential types list pre-populated and the caching
  strategy question. The caching/refresh part is addressed; non-adaptor
  credential types are not.