Fix timezone loss and time shifts in encrypted datetimes

[script3r]

With USE_TZ=True, encrypted datetimes stored by SQLite were decrypted as naive values because binary columns skip Django's datetime result converters. Re-saving a value under a non-UTC default timezone could shift the instant and break deterministic equality lookups.

Restore the connection timezone after decrypting naive datetime representations. Already-aware values and USE_TZ=False retain their behavior. Serialization is unchanged, so existing ciphertext and deterministic equality remain compatible without a data migration.

Validation: 6 new cases failed before the fix. All 102 library tests and 6 example tests pass on Python 3.14 / Django 6.0 and Python 3.10 / Django 5.2 / Tink 1.13.0. Tests cover UTC and non-UTC database timezones, raw ciphertext, existing offset and naive representations, values_list(), re-saves, and deterministic equality. Ruff lint/format and Pyright pass.

Stack position: 2 of 6; depends on #7. Land #7 first, then retarget this PR to main if GitHub has not done so automatically. Database integration was tested with SQLite.

Landing order: #7 → #8 → #9 → #10 → #11 → #12.

Full review and landing notes. Use merge commits to preserve stack ancestry; squash/rebase merges require rebasing the remaining stack before landing it.