fix(deploy): migrate after stop and shrink DB pool

Stage migrate hit ER_CON_COUNT_ERROR while live still held the pool. Build in stage with DATABASE_POOL_SIZE=5, run db:migrate only after stopping the service (with retries), and lower the default pool from 40 to 10.

Co-authored-by: Cursor <[email protected]>

.env.example

@@ -4,7 +4,7 @@
 DATABASE_URL=mysql://user:[email protected]:3306/atomcms
 
 # Optional pool tuning (defaults shown)
-DATABASE_POOL_SIZE=40
+DATABASE_POOL_SIZE=10
 DATABASE_IDLE_TIMEOUT_MS=300000
 DATABASE_CONNECT_TIMEOUT_MS=10000