EpcPropertyModel.uprn was `Optional[int]`, which SQLModel maps to INTEGER, so a
real 11/12-digit UPRN (e.g. 10000452538) overflowed int32 and hard-failed the
insert with NumericValueOutOfRange — a failure on EVERY real-cert save, masked
because the round-trip fixtures use small fake UPRNs. The FE `property.uprn` is
already bigint; pin the column to BigInteger so ours matches (backend-only, no
migration). Verified: uprn 10000452538 saves + reloads.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>