fix(db): make the postgres major-version migration safe and correct

The in-place pg_upgrade in the db entrypoint could crash-loop or fail on real
clusters. This reworks it:

- Idempotent, crash-safe: replace the fragile migration_in_progress marker with
  a state-driven guard on the old_data/new_data scratch dirs. An empty leftover
  means a run was interrupted before any data moved (data still intact at
  $PGDATA) so it is discarded and retried; a non-empty one means data may live
  only there, so it stops for manual recovery. Removes both the
  "mkdir: File exists" crash-loop and the silent fresh-initdb-over-live-data
  window.

- Correct install user: pg_upgrade must run as the old cluster's bootstrap
  superuser (oid 10), and the new cluster must be initialised with that same
  user. It is not necessarily $POSTGRES_USER (clusters created with the default
  "postgres" superuser plus a separate app role are common). Detect it from the
  old cluster (briefly start it and read pg_roles where oid = 10) and use it for
  both the new cluster's initdb and the pg_upgrade -U argument.

- Bump DB_ENTRYPOINT_VERSION v1->v2 so swarm reloads the (immutable) config.

Verified on cctest: clean 13->17, interrupted-then-retried, and prod-like
clusters whose install user is "postgres" with a separate "discourse" app role.

abra.sh

@@ -1,2 +1,2 @@
-export DB_ENTRYPOINT_VERSION=v3
+export DB_ENTRYPOINT_VERSION=v2
 export PG_BACKUP_VERSION=v2

compose.yml

@@ -43,7 +43,7 @@ services:
         #- "traefik.http.routers.${STACK_NAME}.middlewares=${STACK_NAME}-redirect"
         #- "traefik.http.middlewares.${STACK_NAME}-redirect.headers.SSLForceHost=true"
         #- "traefik.http.middlewares.${STACK_NAME}-redirect.headers.SSLHost=${DOMAIN}"
-        - "coop-cloud.${STACK_NAME}.version=0.8.1+3.5.0"
+        - "coop-cloud.${STACK_NAME}.version=0.8.0+3.5.0"
     healthcheck:
       test: "ruby -e \"require 'uri'; require 'net/http'; uri = URI('http://localhost:3000/srv/status'); res = Net::HTTP.get_response(uri); if res.is_a?(Net::HTTPSuccess) then exit (0) else exit (1) end\""
       interval: 30s