[ntttttbl123-blip]

Summary

Acquire a Postgres session-level advisory lock (pg_try_advisory_lock) per keeper job tick to prevent race conditions, duplicate submissions, and sequence number collisions across multiple replicas or during rolling restarts.

Changes

• Export createRawClient and RawSql from @ballast/db for raw SQL connection acquisition.
• Implement PostgresLockProvider and executeJobWithLock in services/keeper/src/lock.ts using dedicated reserved connections and session-level advisory locks.
• Support configurable lock scope via KEEPER_LOCK_SCOPE=job|global (defaulting to job scope: hashtext("keeper:" || name)).
• Wrap execute and runJob in services/keeper/src/index.ts to log and skip ticks when the advisory lock is busy.
• Add unit tests verifying replica concurrency avoidance, lock release on crashed jobs, and scope handling.
• Document replica behaviour and advisory locking in services/keeper/README.md.

Closes #58

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