## Summary
v3 (the engine that ran the SDK v3 era, internally
`RunEngineVersion.V1`) is end-of-life. Following the removal of the v3
execution apps
([#4194](https://github.com/triggerdotdev/trigger.dev/pull/4194)) and
the legacy dev websocket
([#4198](https://github.com/triggerdotdev/trigger.dev/pull/4198)), this
removes the remaining v3 execution stack from the server.
Clients still on v3 (an old SDK or CLI that has not upgraded) keep
getting a clear "upgrade to v4" response. Triggers, batch triggers,
reschedules, and deploys that resolve to v3 are rejected with a graceful
4xx pointing at the migration guide, never a 5xx, so a stale client
cannot affect server health. Self-hosted instances still running v3
should stay on the 4.5.x release line until they migrate.
## What is removed
- The MarQS queue and its shared/dev queue consumers.
- The v3 socket.io namespaces (coordinator, provider, shared-queue) and
the v3 run lifecycle services (attempt, checkpoint, and batch-resume).
- The graphile-worker background job system; all live jobs already run
on `@trigger.dev/redis-worker`.
- The `DEPRECATE_V3_ENABLED` flag: v3 is now rejected unconditionally,
so the flag is gone.
- Unused v3 exports from `@trigger.dev/core` (the `v3/zodNamespace`
subpath and the legacy socket message catalogs) and the now-dead MarQS
environment variables.
## What stays
The v4 engine is untouched. The graceful v3 rejection boundary stays,
`determineEngineVersion` still detects a v3 project so it can reject it,
and the batch service plus batch-completion worker stay for current
clients. Live queue concurrency limits and metrics now read from the v4
run engine instead of MarQS, and a brand-new dev environment now
defaults to v4.
## Dependency cleanup
Removes webapp dependencies left unused by this change: `seedrandom` and
`semver` (only the removed v3 code used them) plus a set that was
already dead, their orphaned `@types` packages, and two dead files. Adds
a `knip:deps` script and a `knip.json` config so unused dependencies can
be found the same way going forward.
Chris Arderne