Improvement: Rename dev-engine "Video toolchain ready" to a general external-toolchain label

[yannickmonney]

Problem

bun / docker:dev progress prints “Video toolchain ready” for a step that is really “provision host/external binaries the stack needs.” The video-specific name undersells the step and will age poorly as more non-video tools land in the same provision path.

Current behaviour

In services/platform/scripts/dev-engine.ts (~L1172–1177):

await runStep(
  {
    active: 'Provisioning video toolchain',
    done: 'Video toolchain ready',
  },
  provisionVideoToolchain,
);

The step installs/resolves yt-dlp + deno + ffmpeg (and related paths) before Convex env sync. Failure copy nearby still says “Video toolchain provisioning failed…”.

Proposed change

• Rename the progress strings to a general external-toolchain label, e.g.:
  • active: Provisioning external toolchains
  • done: External toolchains ready
• Keep the underlying provisionVideoToolchain function name for now unless a rename is cheap; if the helper grows beyond video, rename the symbol in the same change.
• Update any matching failure / log strings so the UX is consistent.

Acceptance criteria

• Dev-engine progress no longer says “Video toolchain ready” / “Provisioning video toolchain”.
• New copy reads as general external/host toolchains (not video-only).
• Failure messages match the new naming.

Affected service

Platform (dev tooling)