feat(worker): add X-Entity-Id header to job dispatch pipeline

[pyramation]

Summary

Promotes entity_id from a buried payload field to a first-class HTTP header (X-Entity-Id) across the entire job dispatch pipeline:

1. Worker (job/worker/src/index.ts) — entity_id added to JobRow interface, read from app_jobs.jobs column
2. Dispatch (job/worker/src/req.ts) — emits X-Entity-Id header alongside X-Database-Id and X-Actor-Id
3. fn-types (packages/fn-types/src/runtime.ts) — entityId added to FunctionContext.job
4. fn-runtime (packages/fn-runtime/src/context.ts, server.ts) — reads X-Entity-Id header into context
5. fn-app (packages/fn-app/src/index.ts) — reads into JobContext, echoes on responses, forwards in callbacks

This is Phase 1 of the cloud functions metering architecture. With entity_id available in function context and callbacks, the metering layer can bill compute to the correct entity (org/team within a customer app).

Depends on the companion constructive-db PR which adds the entity_id column to app_jobs.jobs.

Review & Testing Checklist for Human

• Verify X-Entity-Id header is correctly emitted when entity_id is present on the job row (and omitted when NULL — conditional spread preserves backward compat)
• Verify context.job.entityId is accessible in function handlers
• Verify the callback to job/server includes X-Entity-Id so the completion handler has entity context for metering

Notes

Backward compatible — when entity_id is NULL/undefined, no header is emitted (same conditional spread pattern as X-Database-Id and X-Actor-Id). Existing functions that don't use context.job.entityId are unaffected.

Link to Devin session: https://app.devin.ai/sessions/8820b639fbd94ac8bfae86cfb01cf827
Requested by: @pyramation

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