perf(ip-sync): avoid redundant client IP reads and writes (#6930)

The access-log scanner and the cross-node merge stored the same IP list in
opposite orders, so neither saw a row as unchanged, and the merge re-read
every preloaded row and re-upserted every attribution row each sync cycle.
All writers now share one canonical encoder (timestamp, then IP ascending),
the first CAS is seeded from the preload, and identical attribution rows are
skipped. A create race between the scanner and node sync now merges into the
winning row instead of dropping the loser's IPs, which on PostgreSQL also
aborted the scanner's whole scan transaction.

Closes #6927

.github/workflows/ci.yml

@@ -90,6 +90,16 @@ jobs:
             exit 1
           fi
 
+      - name: PostgreSQL client IP sync regressions
+        run: |
+          set -o pipefail
+          go test -p=1 ./internal/web/service ./internal/web/job -run '^TestClientIpSyncPostgres(Scanner)?$' -count=1 -v | tee /tmp/postgres-client-ip-sync.log
+          passed=$(grep -c -- '^--- PASS' /tmp/postgres-client-ip-sync.log || true)
+          if [ "$passed" -ne 2 ]; then
+            echo "expected 2 passing PostgreSQL client IP sync tests, got $passed" >&2
+            exit 1
+          fi
+
       - name: PostgreSQL global-traffic heartbeat regressions
         run: |
           set -o pipefail