fix(db-tool): postgres-init: createdb on reuse when target DB missing

Previously when PG_REUSE=1 and PG_VERSION existed but the target database had
never been successfully created (e.g. devshell exited mid-init in a prior run),
postgres-init skipped createdb and the subsequent psql connection failed with
'database "<db>" does not exist'.

Now on reuse path we probe pg_database and create the target DB if missing,
making postgres-init fully idempotent across stale-state recovery.

Adds postgres-init-reuse-missing-db test.

test/package/db-tool/test/postgresql/postgres-init-reuse-missing-db/run.sh

@@ -0,0 +1,45 @@
+# shellcheck shell=dash
+
+HECTIC_NAMESPACE=test-db-tool-postgres-init-reuse-missing-db
+
+PG_WORKING_DIR=$(mktemp -d)
+export PG_WORKING_DIR PG_DATABASE=testdb PG_PORT=5432 PG_SHARED_PRELOAD_LIBRARIES=''
+
+cleanup() {
+  postgres-cleanup >/dev/null 2>&1 || :
+  rm -rf "$PG_WORKING_DIR"
+}
+trap 'cleanup' EXIT INT TERM
+
+log notice "step 1: fresh init creates testdb"
+if ! postgres-init; then
+  log error "initial postgres-init failed"
+  exit 1
+fi
+
+log notice "step 2: drop testdb to simulate stale-state cluster"
+sockdir="$PG_WORKING_DIR/sock"
+if ! dropdb -h "$sockdir" -p "$PG_PORT" -U "$(id -un)" testdb; then
+  log error "dropdb failed"
+  exit 1
+fi
+
+log notice "step 3: stop cluster (simulate prior devshell exit)"
+postgres-cleanup >/dev/null 2>&1 || :
+
+log notice "step 4: re-init with PG_REUSE=1 must recreate missing testdb"
+export PG_REUSE=1
+if ! postgres-init; then
+  log error "postgres-init with PG_REUSE=1 and missing DB failed"
+  exit 1
+fi
+unset PG_REUSE
+
+log notice "step 5: verify testdb is reachable"
+pgurl="postgresql://$(id -un)@/testdb?host=${sockdir}&port=5432"
+if ! psql "$pgurl" -c 'SELECT 1;' >/dev/null 2>&1; then
+  log error "testdb not reachable after reuse-create"
+  exit 1
+fi
+
+log notice "test passed"