fix(db): make pg_upgrade migration idempotent & crash-safe

The postgres major-version migration in the db entrypoint was not safe to
re-run. If the container was killed mid-migration it could crash-loop forever
("mkdir: cannot create directory .../old_data: File exists") or silently initdb
a fresh empty cluster over the live data once PG_VERSION had been moved out of
$PGDATA but before the in-progress marker was written.

Replace the marker file with a state-driven guard keyed on the scratch dirs:
empty old_data/new_data means the run was interrupted before any data moved, so
discard and retry (idempotent); non-empty means data may only live there, so
stop for manual recovery. Bump DB_ENTRYPOINT_VERSION v1->v2 so swarm picks up
the new (immutable) config.

abra.sh

@@ -1,2 +1,2 @@
-export DB_ENTRYPOINT_VERSION=v1
+export DB_ENTRYPOINT_VERSION=v2
 export PG_BACKUP_VERSION=v2

entrypoint.postgres.sh.tmpl

@@ -2,16 +2,23 @@
 
 set -e
 
-MIGRATION_MARKER=$PGDATA/migration_in_progress
 OLDDATA=$PGDATA/old_data
 NEWDATA=$PGDATA/new_data
 
 echo "Running as $(id)"
 
-if [ -e $MIGRATION_MARKER ]; then
-  echo "FATAL: migration was started but did not complete in a previous run. manual recovery necessary"
-  exit 1
-fi
+# The migration uses $OLDDATA/$NEWDATA as scratch and removes them when it
+# finishes; a leftover *empty* one means a run was interrupted before any data
+# moved (data still intact at $PGDATA) so we clear it and retry, while a
+# *non-empty* one means data may live only there, so we stop for manual recovery.
+for scratch in $OLDDATA $NEWDATA; do
+  if [ -d "$scratch" ] && [ -n "$(ls -A "$scratch")" ]; then
+    echo "FATAL: $scratch exists and is not empty - a previous migration did not"
+    echo "complete and the data may only exist there. manual recovery necessary."
+    exit 1
+  fi
+done
+rm -rf $OLDDATA $NEWDATA
 
 if [ -f $PGDATA/PG_VERSION ]; then
   DATA_VERSION=$(cat $PGDATA/PG_VERSION)
@@ -27,7 +34,6 @@ if [ -f $PGDATA/PG_VERSION ]; then
     gosu postgres mkdir $OLDDATA $NEWDATA
     chmod 700 $OLDDATA $NEWDATA
     mv $PGDATA/* $OLDDATA/ || true
-    touch $MIGRATION_MARKER
     echo "running initdb"
     # abuse entrypoint script for initdb by making server error out
     gosu postgres bash -c "export PGDATA=$NEWDATA ; /usr/local/bin/docker-entrypoint.sh --invalid-arg || true"
@@ -38,7 +44,6 @@ if [ -f $PGDATA/PG_VERSION ]; then
     mv $NEWDATA/* $PGDATA
     rm -rf $OLDDATA
     rmdir $NEWDATA
-    rm $MIGRATION_MARKER
     echo "migration complete"
   fi
 fi