[lehni]

Strict insertGraph() and upsertGraph() (#429), including their *AndFetch variants, now always run in a transaction. A database error in a later write, e.g. a UniqueViolationError, then leaves none of the earlier writes behind.

• The call starts its own transaction only if the query isn't already in one. A query that has a transaction passed to it, or whose model is bound to one, uses that transaction without a savepoint.
• The transaction covers the whole call: the loads of upsertGraph(), the writes and the re-fetch of the *AndFetch variants. Hooks get it as context.transaction.
• onError() handlers run after the rollback, on the query's own connection.
• There's no opt-out. Hooks that run queries outside context.transaction wait for the transaction, and deadlock on SQLite with its single connection, so they need to use context.transaction. The docs say so.

Part of #477: non-strict graphs are unchanged, and making them atomic by default stays for the next major.