fix: Check database connectivity only once

To avoid strain on the database, check connectivity only once on instance start (`lazy val`).
We should see the log line exactly once per instance, regardless of how often the healthcheck endpoint is queried.

Co-authored-by: Julia <JuliaBrigitte@users.noreply.github.com>

Author: akash1810

common/src/main/scala/db/SqlRegistrationRepository.scala

@@ -82,6 +82,7 @@ class SqlRegistrationRepository[F[_]: Async](xa: Transactor[F])
    * We just select one topic and a constant value as this is sufficient to check connectivity (we don't care about what data is returned).
    */
   override def simpleSelectForHealthCheck(): Stream[F, TopicCount] = {
+    logger.info("Performing a query to check DB connectivity")
     sql"""
          SELECT  topic
                 , 1

registration/app/registration/controllers/Main.scala

@@ -33,10 +33,11 @@ final class Main(
 
   private val logger: Logger = LoggerFactory.getLogger(this.getClass)
 
+  // Check if we can talk to the registration database
+  private lazy val dbConnectivityCheck = registrar.dbHealthCheck()
 
   def healthCheck: Action[AnyContent] = Action.async {
-    // Check if we can talk to the registration database
-    registrar.dbHealthCheck()
+    dbConnectivityCheck
       .map(_ => {
         // This forces Play to close the connection rather than allowing
         // keep-alive (because the content length is unknown)