feat(lagoon): per-query collation for OrderBy, drop the database locale check

- lagoon.OrderBy takes variadic lagoon.OrderOption values; lagoon.Collate(name)
  emits a validated, double-quoted COLLATE clause (e.g. "pl-x-icu")
- remove the exported CheckLocale and the ICU pl-PL check from Open and Use
- framework test containers and per-test databases are plain PostgreSQL
- lagoon README, root README and docs pages drop the locale requirement;
  queries-and-pagination gains a "Sorting with a collation" section
  backed by ExampleCollate

docs/plugins/testing.md

@@ -31,10 +31,9 @@ Most plugin code runs without a database. Build a container with `backpack.New`,
 
 ## Database tests
 
-SummerCMS supports PostgreSQL only, so database tests run against real PostgreSQL rather than an SQLite stand-in. The framework's own tests start a `postgres:16-alpine` container through testcontainers-go and create the database with the ICU `pl-PL` locale that lagoon checks for when it connects. Follow the same pattern in plugin tests:
+SummerCMS supports PostgreSQL only, so database tests run against real PostgreSQL rather than an SQLite stand-in. The framework's own tests start a `postgres:16-alpine` container through testcontainers-go and create a fresh database per test. Follow the same pattern in plugin tests:
 
 - skip the test when `testing.Short` reports true;
-- create the database with `LOCALE_PROVIDER icu ICU_LOCALE 'pl-PL'`, or lagoon refuses the connection;
 - migrate a fresh database per test with `lagoon.Migrate`, so tests do not depend on each other.
 
 Docker must be running for these tests.