fix(cabana): search and sort to-many relation columns through subqueries

- relationSubquery reads the pivot, key and table names from GORM's parsed
  relationship (Schema.Relationships.Relations), never from a convention
- list search matches every relation column through a correlated EXISTS,
  so the count query gains no JOIN and belongsToMany search stops failing
- sorting a hasMany or belongsToMany column orders by a correlated
  string_agg subquery before the primary-key tiebreak
- polymorphic relations and unknown related columns answer 422
- DryRun smoke test TestListRelationSearchSmoke needs no Docker

docs/backend/lists-and-filters.md

@@ -59,7 +59,7 @@ columns:
         type: datetime
 ```
 
-A column takes `label`, `type` (`text`, `datetime`, `date`, `time` or `switch`), `searchable` (default false) and `sortable` (default true, as in WinterCMS), and, for a column read through a relation, `relation` and `select`. Other WinterCMS column types and options are refused at boot. When `type` is omitted, a `time.Time` column is `datetime`, a `lagoon.Date` column is `date` and a `lagoon.TimeOfDay` column is `time`; every other column stays untyped (the admin treats that as text). An explicit `type` always wins. The compiled list keeps the columns in file order:
+A column takes `label`, `type` (`text`, `datetime`, `date`, `time` or `switch`), `searchable` (default false) and `sortable` (default true, as in WinterCMS), and, for a column read through a relation, `relation` and `select`. Other WinterCMS column types and options are refused at boot. When `type` is omitted, a `time.Time` column is `datetime`, a `lagoon.Date` column is `date` and a `lagoon.TimeOfDay` column is `time`; every other column stays untyped (the admin treats that as text). An explicit `type` always wins. A searchable relation column matches a record when any related record matches: a `categories` column with `relation: categories` and `select: name` on a blog post finds the posts with at least one matching category, for belongsTo, hasOne, hasMany and belongsToMany relations alike, each through a subquery, so a post is listed and counted once. A sortable hasMany or belongsToMany column sorts by its comma-joined values, the text the list shows. The compiled list keeps the columns in file order:
 
 ```go src=modules/cabana/example_test.go#ExampleCompileList
 // The plugin embeds its controllers/ and models/ trees; the example reads