fix: wait for postgres healthcheck before running migrator

- Add pg_isready healthcheck to db service (5s interval, 10 retries)
- Change migrator depends_on condition: service_started -> service_healthy
- Prevents migrator connection failure on fresh postgres startup

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docker/environments/docker-compose.yml

@@ -9,7 +9,7 @@ services:
       - App__ConnectionString=Host=db;Port=5432;Database=${DB_NAME:?DB_NAME is required};Username=${DB_USER:?DB_USER is required};Password=${DB_PASS:?DB_PASS is required}
     depends_on:
       db:
-        condition: service_started
+        condition: service_healthy
     restart: "no"
 
   api:
@@ -72,6 +72,11 @@ services:
       - pgdata:/var/lib/postgresql/data
     networks:
       - internal
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U ${DB_USER}"]
+      interval: 5s
+      timeout: 5s
+      retries: 10
 
 volumes:
   pgdata: