feat(deploy): bake the ServerCluster/ClusterNode seed into docker-compose

Adds a one-shot cluster-seed service to docker-dev/docker-compose.yml
that pre-populates the three Akka clusters' scope rows in the shared
OtOpcUa ConfigDb so operators don't have to click through /clusters +
/hosts on every fresh bring-up.

Seed contents:
  ServerCluster   MAIN (Warm/2), SITE-A (Warm/2), SITE-B (Warm/2)
  ClusterNode     driver-a + driver-b  → MAIN
                  site-a-1 + site-a-2  → SITE-A
                  site-b-1 + site-b-2  → SITE-B

NodeCount + RedundancyMode honour the CK_ServerCluster check constraint.
ApplicationUri follows the urn:OtOpcUa:<NodeId> convention; uniqueness
across the fleet satisfies UX_ClusterNode_ApplicationUri.

Mechanism:
  - docker-dev/seed/seed-clusters.sql — idempotent INSERTs (IF NOT EXISTS
    guards on every row).
  - docker-dev/seed/entrypoint.sh — bash wrapper that waits for SQL to
    accept connections, then polls until dbo.ServerCluster exists (the
    host containers' EF auto-migration creates it on first boot), then
    applies the SQL script.
  - cluster-seed service uses mcr.microsoft.com/mssql-tools as the base
    image (bash + sqlcmd available), restart: "no" so it runs once.

Re-running `docker compose up` is safe: the seed exits cleanly on the
second run because every INSERT is guarded.

Manual re-seed: `docker compose run --rm cluster-seed`.

docker-dev/seed/entrypoint.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+# docker-dev cluster-seed entrypoint. Waits for the host containers to finish
+# their EF Core auto-migration (which creates the ServerCluster table), then
+# applies the idempotent seed script.
+#
+# Image: mcr.microsoft.com/mssql-tools (Debian + sqlcmd at /opt/mssql-tools18/bin).
+
+set -euo pipefail
+
+SQLCMD="/opt/mssql-tools18/bin/sqlcmd"
+SERVER="${SQL_HOST:-sql},1433"
+USER="${SQL_USER:-sa}"
+PASS="${SQL_PASSWORD:-OtOpcUa!Dev123}"
+DB="${SQL_DATABASE:-OtOpcUa}"
+
+run_sql() {
+    "$SQLCMD" -S "$SERVER" -U "$USER" -P "$PASS" -d "$DB" -No -b -h -1 "$@"
+}
+
+echo "[cluster-seed] waiting for SQL Server to accept connections..."
+until run_sql -Q "SELECT 1" >/dev/null 2>&1; do
+    sleep 2
+done
+echo "[cluster-seed] SQL Server up."
+
+echo "[cluster-seed] waiting for $DB.ServerCluster (host containers must finish EF migration)..."
+until run_sql -Q "IF OBJECT_ID('dbo.ServerCluster') IS NULL THROW 50001, 'missing', 1; SELECT 1" >/dev/null 2>&1; do
+    sleep 3
+done
+echo "[cluster-seed] schema ready."
+
+echo "[cluster-seed] applying seed-clusters.sql..."
+run_sql -i /seed/seed-clusters.sql
+
+echo "[cluster-seed] done."