ci(postgres): fail setup if pg_ctl stop fails before baking the datadir

The "Stop DB and stage datadir" step swallowed a failed `pg_ctl -m fast -w
stop` with `|| true`, then moved and tarred the PGDATA regardless. A stop
that times out or errors would bake a still-running, crash-inconsistent
cluster into the artifact every test shard consumes — and with
full_page_writes off, crash recovery can't repair torn pages. Drop the
`|| true` so a failed stop fails the job, mirroring the MariaDB sister's
"don't bake a dirty datadir" guard.

Also drop the redundant `ALTER SYSTEM SET fsync/synchronous_commit/
full_page_writes = off` block from install.sh. Its comment claimed the
postgres workflow "runs a service-container DB and never calls start-db.sh",
but it does call start-db.sh, which already applies those flags via `-o` on
every postgres start (setup job and each shard). The block was a no-op and
its justification was factually wrong.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/server-tests-postgres.yml

@@ -108,7 +108,11 @@ jobs:
       - name: Stop DB and stage datadir
         run: |
           PG_BIN=$(ls -d /usr/lib/postgresql/*/bin | sort -V | tail -1)
-          "$PG_BIN/pg_ctl" -D /home/runner/pgdata -m fast -w stop || true
+          # Clean shutdown so the baked datadir is consistent. Do NOT swallow a failed stop with
+          # `|| true`: moving and tarring a still-running cluster ships a torn datadir the shards
+          # cannot crash-recover (full_page_writes is off). Fail the job instead — mirrors the
+          # MariaDB sister's "don't bake a dirty datadir" guard.
+          "$PG_BIN/pg_ctl" -D /home/runner/pgdata -m fast -w stop
           mv /home/runner/pgdata /home/runner/frappe-bench/pgdata
 
       - name: Package bench for test shards