db: fix intvars being truncated to 32 bits on write and read

[Andezion]

Fixes #9467

What changed

• db/exec.c - db_set_intvar, db_get_intvar, and db_data_version_get (reads the same column) now use the existing 64-bit db_bind_s64/db_col_s64 helpers instead of the 32-bit ones

• wallet/migrations.c - new migration widens vars.intval to BIGINT on Postgres only (/PSQL/ no-op on SQLite, which doesnt enforce column width), with a matching revert

• wallet/test/run-db.c - added a regression case round-tripping a value >32 bits, verified it fails on the old code and passes with the fix

Changelog-Fixed: db: fixed vars table integers being truncated to 32 bits

Important

26.09 FREEZE August 5th: Non-bugfix PRs not ready by this date will wait for 26.12.

RC1 is scheduled on August 17th

The final release is scheduled for September 7th.

Checklist

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• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes.
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.
• Important All PRs must consider how to reverse any persistent changes for tools/lightning-downgrade