ci: tune Postgres durability in pg test jobs

[pyramation]

Summary

Adds a Tune Postgres for CI (disable durability) step before Seed app_user in the three Postgres-backed jobs (pg-tests, integration-tests, ai-tests) of run-tests.yaml. Solves the self-referential case by invoking the workspace-built CLI — pnpm --filter pgpm exec node dist/index.js tune --yes — which exists since #1336 and is built by the upstream build job, so no npm release dependency.

Tuning applies fsync/synchronous_commit/full_page_writes = off, checkpoint_timeout = 30min, max_wal_size = 8GB via ALTER SYSTEM + pg_reload_conf(), eliminating checkpoint I/O stalls on CI runners (root cause of flakiness fixed in constructive-io/constructive-db#1925).

Link to Devin session: https://app.devin.ai/sessions/7943c7526d254aa4828be110fbb3f272
Requested by: @pyramation

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