Introduce the multi-tenant Space concept without changing any behaviour yet:
every user gets an invisible personal space provisioned on creation, and all
owned models (accounts, connections, transactions, categories, labels, budgets,
automation rules, saved filters) gain a nullable space_id stamped via a
BelongsToSpace trait. An idempotent, chunked spaces:backfill command (run from a
migration) stamps existing rows so space-scoped reads can go live safely.
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