refactor(queries): replace hardcoded event types with constants

Replaced hardcoded `event_type` values with named constants from
`EVENT_TYPE`, as in #3614. Added the `BOUNCE_THRESHOLD` constant and
documented it in `docker-compose.yml`.

docker-compose.yml

@@ -7,6 +7,7 @@ services:
     environment:
       DATABASE_URL: postgresql://umami:umami@db:5432/umami
       APP_SECRET: replace-me-with-a-random-string
+      # BOUNCE_THRESHOLD: 2 # OPTIONAL: set the minimum amount custom events to trigger a bounce
     depends_on:
       db:
         condition: service_healthy