Add heartbeat to background worker epoch sentinel

[Pino de Candia (pinodeca)]

Summary

Add a last_seen_at column to df._worker_epoch and update the epoch sentinel check to write a heartbeat on every poll tick (~5 seconds). This gives operators a simple SQL query to verify background worker liveness without inspecting logs or process lists.

Changes

• src/lib.rs: Add last_seen_at TIMESTAMPTZ column to df._worker_epoch DDL
• src/worker.rs: Update sentinel insert to include last_seen_at; replace SELECT EXISTS(...) with UPDATE ... SET last_seen_at = now() ... RETURNING epoch_id
• USER_GUIDE.md: Add Worker Liveness subsection to Monitoring
• docs/extension_lifecycle.md: Update schema description to include new column
• tests/e2e/sql/35_heartbeat_liveness.sql: E2E test that verifies last_seen_at advances over time
• scripts/test-e2e-local.sh: Register new test as superuser (reads internal df._worker_epoch)

User-Facing Liveness Check

SELECT epoch_id, started_at, last_seen_at,
       now() - last_seen_at AS time_since_last_heartbeat
  FROM df._worker_epoch;

• time_since_last_heartbeat < 15 seconds → worker is alive (recent heartbeat)
• No rows or large gap → worker is likely down or hasn't initialized yet

Cost

• No new queries — the existing epoch-check SELECT is replaced with an UPDATE ... RETURNING (same round trip)
• One additional column written per poll tick (~5s) — negligible I/O

[Copilot]

Pull request overview

Adds a heartbeat timestamp to the background worker epoch sentinel so operators can query worker liveness via SQL, without relying on logs or process inspection.

Changes:

• Extend df._worker_epoch with a last_seen_at column.
• Make the epoch-sentinel poll update last_seen_at (heartbeat) on each tick via UPDATE ... RETURNING.
• Document the new liveness query in the user guide and lifecycle docs.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/worker.rs	Writes last_seen_at on sentinel creation and updates it on each poll tick.
src/lib.rs	Adds last_seen_at TIMESTAMPTZ to df._worker_epoch table DDL.
USER_GUIDE.md	Adds a “Worker Liveness” section with a query and interpretation guidance.
docs/extension_lifecycle.md	Updates epoch-sentinel schema description to include last_seen_at.