fix(11-07): hand after-commit callbacks a clean statement

- lagoon.Transaction, the lagoon:after_commit flush and the immediate
  AfterCommit path pass a handle with an empty statement on the write's
  connection (Session NewDB+Context, Clauses(), Session NewDB)
- a WithContext query through the handle no longer continues from the
  written model's statement (deferred from 11-05)
- TestTransactionAfterCommit/callback_handle_has_a_clean_statement covers
  the implicit, plain-transaction and lagoon.Transaction paths

modules/lagoon/README.md

@@ -14,7 +14,7 @@ Postgres data layer: the shared GORM connection, per-plugin migrations, model he
 
 - One shared pool: `lagoon.Open`, `lagoon.Use` and `lagoon.OpenFromApp` return a `*sql.DB` and a `*gorm.DB` built on that same pool; `lagoon.Publish` makes both available on the `backpack.App`.
 - Database-ready hooks: `lagoon.OnDatabase` runs a callback with the pool and GORM handle as soon as the database is published, immediately when it already is, otherwise when `lagoon.Publish` runs. Plugins register GORM callbacks through it from Boot, which runs before the `serve` command publishes the database.
-- After-commit work: `lagoon.Transaction` runs a function in a transaction and then the callbacks registered with `lagoon.AfterCommit`, in order, only after the commit succeeds; a nested `lagoon.Transaction` is a savepoint whose callbacks are dropped with it when it fails. A single-statement write for which GORM opens its own transaction runs its `lagoon.AfterCommit` callbacks from the `lagoon:after_commit` GORM callback (`lagoon.AfterCommitCallback`) once GORM commits, and never when the write fails. Outside both, including inside a plain GORM `Transaction`, `lagoon.AfterCommit` runs the callback immediately. A panicking callback is logged and never turns a committed write into an error.
+- After-commit work: `lagoon.Transaction` runs a function in a transaction and then the callbacks registered with `lagoon.AfterCommit`, in order, only after the commit succeeds; a nested `lagoon.Transaction` is a savepoint whose callbacks are dropped with it when it fails. A single-statement write for which GORM opens its own transaction runs its `lagoon.AfterCommit` callbacks from the `lagoon:after_commit` GORM callback (`lagoon.AfterCommitCallback`) once GORM commits, and never when the write fails. Outside both, including inside a plain GORM `Transaction`, `lagoon.AfterCommit` runs the callback immediately on that transaction's connection. The handle a callback receives always has an empty statement on the connection its work belongs to, so a query through it, even one that starts with `WithContext`, never continues from the written model's statement. A panicking callback is logged and never turns a committed write into an error.
 - Database check at connect time: `lagoon.CheckLocale` refuses a database whose default collation is not the ICU `pl-PL` locale, so ordering matches the database default without per-query `COLLATE`.
 - Per-plugin migrations: `lagoon.Migrate` runs the framework's `system_files` set (`attach.Migrations`), backend admin identity set (`lagoon.BackendAdminMigrations`) and job-queue set (`lagoon.QueueMigrations`: River's schema pinned at `lagoon.RiverSchemaVersion`, then the `lagoon.JobsTable` record table, under the `lagoon.QueueHistoryID` history), then every `pact.HasMigrations` set in plugin activation order, each in its own `summer_migrations_<plugin_id>` history table (`lagoon.HistoryTableName`). `lagoon.RollbackLast` and `lagoon.Status` cover rollback and history.
 - Mass assignment: `lagoon.Fill` copies only allow-listed keys onto a model by GORM column name and silently drops the rest, logging each dropped key once outside production. A `json.Number` (from a decoder using `UseNumber`) fills integer, unsigned and float fields. A value that does not fit its column (a fraction, an exponent or an overflow for an integer field, or a value of the wrong type) is a `lagoon.FillTypeError` naming the key, so a caller can answer it as a validation failure on that field. `lagoon.HasFillable` and `lagoon.HasHidden` are the Go forms of `$fillable` and `$hidden`.