Postgres session-advisory-lock implementation of ALLM.Pipeline.Lock.

PRESERVED, NOT ACTIVE. The active implementation is ALLM.Pipeline.Lock.Noop — see ALLM.Pipeline.Lock for why the lock was dropped and how to restore this one. This module is kept intact so the guarantee can be brought back deliberately rather than reconstructed from scratch. The serialization mapping it applies is no longer its own — batch 1.C moved it onto the host's ALLM.Pipeline.Registry (lock_keys:); the host-side membership guard is a consumer repo's registry-declared-values test, with its runner test pinning the derived lock keys.

Mechanism

Scheduled cron runs of the same pipeline can overlap if a previous run is slow (e.g. an ordinance pipeline still grinding through PDFs when the next weekly trigger fires). To prevent overlap on the shared Postgres database, each invocation acquires a session-scoped advisory lock keyed on the pipeline name.

• We use pg_try_advisory_lock(key) (NOT the _xact_ variant) so the lock survives across the many transactions inside a long pipeline.
• The lock is held by a single dedicated connection, checked out via Ecto.Repo.checkout/2. pg_try_advisory_lock is bound to the connection that ran it; releasing on a different connection is silently a no-op, so we MUST run lock + work + unlock on the same checked-out connection. This pinned connection is exactly the fragility that motivated the switch to Noop — it sits idle during long LLM-bound steps and can be reaped.
• The key is derived from the pipeline name (:erlang.phash2/1), stable within a process lifetime and small enough for Postgres's bigint. Different pipelines get different keys, so unrelated pipelines run concurrently. EXCEPTIONS collapse to one key via canonical_lock_name/1.
• On contention pg_try_advisory_lock returns false and we return {:error, :already_running} immediately, without invoking the pipeline.
• try/after releases the lock on every exit path, including raises. An advisory lock is also session-bound, so even a hard crash releases it when the connection dies.