store: Use REPEATABLE READ transaction for consistent dump

Wrap the dump's data-reading operations in a REPEATABLE READ READ ONLY
transaction to get a consistent MVCC snapshot. This prevents head block
vs entity data mismatches, cross-table inconsistency, and missing or
phantom rows caused by concurrent indexing or pruning.

Add a TransactionBuilder for PermittedConnection since diesel-async's
TransactionBuilder requires TransactionManager = AnsiTransactionManager,
which pool-wrapped connection types don't satisfy.