Land the `name` field and `next_definition_sequence` counter in the
two real persistence backends. Both providers:
* Add `name TEXT NOT NULL UNIQUE` to the `workflows` table.
* Add a `definition_sequences` table (`definition_id, next_num`) with
an atomic UPSERT + RETURNING to give the host a race-free monotonic
counter for `{def_id}-{N}` name generation.
* INSERT/UPDATE queries now include `name`; SELECT row parsers hydrate
it back onto `WorkflowInstance`.
* New `get_workflow_instance_by_name` method for name-based lookups
used by grpc handlers.
Postgres includes a DO-block migration that back-fills `name` from
`id` on pre-existing deployments so the NOT NULL + UNIQUE invariant
holds retroactively; callers can overwrite with a real name on the
next persist.
PostgresPersistenceProvider using sqlx with wfc schema. JSONB for
complex fields, TIMESTAMPTZ for dates. Transactions for atomicity.
Proper error propagation on deserialization (no unwrap_or_default).
Status conversion returns Result for unknown values.
