Dashboard charts (activity, message-activity, node-count) rendered as
flat zeros on Postgres because func.date() returns a str on SQLite but
a datetime.date on Postgres — the dict lookup by string key always
missed. Fixed with a dialect-neutral _date_bucket_key() helper and
pinned the Postgres session timezone to UTC at the engine level.
Also adds dual-backend test infrastructure (TEST_DATABASE_BACKEND env
var), per-worker Postgres databases for pytest-xdist isolation, and
strengthened regression tests asserting non-zero date buckets.
Louis King
cf5add9924