Don't allow null speakers on statements

- All new statements should be added with a speaker, and old statements without one should be migrated.
- Constraint should not be enforced in DB in a first time as we have existing statements with nil speakers.
- Frontend can be implemented first, independently of backend.

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:information_source: Some work was started on the frontend by @julien-leclercq [here](https://github.com/CaptainFact/captain-fact-frontend/pull/176) but PR was dropped after being staled for too long in a buggy state. Can be used as reference, but conflicts now prevent the code to be re-used.

Review the issue requirements and the abandoned captain-fact-frontend PR #176 first, treating it as reference only because the issue says it is buggy and conflicts prevent reuse. Trace statement creation, existing nil-speaker records, and the frontend statement flow. Done means new statements have speakers, old records are migrated, and the database constraint remains deferred initially.

Written by the indexing model from the issue text.