Introduce automatic healing for stale PDF parse states in API routes. Add
`isDocumentParseStateStale` utility and use a shared `COMPUTE_OP_STALE_MS`
environment variable (with config getter) to control the stale window for both
worker op replacement and app-side parse-state healing. Update documentation and
environment variable references to reflect new config.
Enables automatic marking of stuck parses as failed, improving reliability and
retry behavior in distributed compute environments.
09e8898683