fix(library-identity-consumer): log error.cause on writer failure so PG diagnostics surface

[jakebromberg]

Summary

• Drizzle wraps every postgres-js failure in `DrizzleQueryError`, whose `.message` is hardcoded to `Failed query: \nparams: ` (drizzle-orm/errors.cjs:35-44). The actual PG error (`code`, `detail`, `constraint_name`, `column_name`) lives on `.cause`.
• The orchestrator's catch block only captured `(error as Error).message`. During the 2026-05-20 first prod run of `library-identity-consumer`, every one of the 14,405 `single_artist` resolves failed at write, producing 14,405 identical `Failed query: INSERT INTO ...library_identity_source...` log lines with no PG diagnostic. Diagnosis required a manual psql repro.
• Patch: extend the `warn` log line to also surface `error.cause`'s standard PostgresError properties (`pg_message`, `pg_code`, `pg_detail`, `pg_constraint`, `pg_column`, `pg_table`, `pg_routine`).
• Diff is +19/-1 in a single file. Type-check clean against the consumer's tsconfig.

Why this is independent of the underlying writer_error root cause

Whatever fixes the actual writer failure (TBD; likely a constraint or column-shape issue surfaced by the 2026-05-20 run), the lesson stays: silently swallowing `error.cause` makes any future Drizzle-wrapped DB failure undebuggable from logs alone. This change is observability hygiene, not a behavior change.

Test plan

• `npx tsc --noEmit -p jobs/library-identity-consumer/tsconfig.json` clean.
• No new tests — the catch block is logging-only; behavior of the surrounding orchestration is unchanged.
• Manual verification on next `library-identity-consumer` re-run: warn lines should include `pg_code`, `pg_constraint` etc. when `error.cause` is a PostgresError.

Followup

This unblocks the diagnosis path on #802's first-run writer failure. Underlying root cause (and a separate fix) tracked separately once the PG error surfaces.