T53716: Add DurationField support (serialization, filtering, tests)

[SotirisDimitrakoulakos]

Initial Problem — Models with a Django models.DurationField were unusable through the API:

• any GET/POST/PUT response containing a duration led to 500 (TypeError: datetime.timedelta is not JSON serializable)
• filtering on a duration field led to 418 (no filter class for the field type)
• writing only worked on PostgreSQL (whose interval parser tolerates the raw string); on MySQL/SQLite every duration write crashed at save()

Changes

• binder/json.py: register datetime.timedelta in SERIALIZERS - django.utils.duration.duration_string. Wire format: "[D] [HH:]MM:SS[.uuuuuu]" (e.g. 00:30:00, 2 03:04:05, -1 23:30:00) — exactly what parse_duration/to_python accept, so GET output round-trips on PUT.
• binder/models.py: new DurationFieldFilter (auto-registered via FieldFilter.__subclasses__()), parses with django.utils.dateparse.parse_duration, same qualifier set as the Date/DateTime/Time filters (in, gt, gte, lt, lte, range, isnull); invalid value leads to 418 RequestError like all other filters.
• binder/views.py: the generic write path (_store_field) now stores the parsed value for DurationField instead of the raw string. Previously to_python() was already called (for validation) but its result was discarded and the raw string was stored — which only works on PostgreSQL (its interval parser tolerates the string at save time). On MySQL/SQLite, get_db_prep_value() leads to duration_microseconds(), performs attribute access on a timedelta and crashes on a str, so every duration write 500s on those backends.
• Tests: new Truck test model + view (one DurationField, one nullable) and 12 new tests — serializer unit test (4 formats incl. multi-day and negative), POST/GET/PUT round-trip, default value, 400/418 write error paths, and exact/gte/range/invalid filter tests.
• changelog/T53716

[knokko]

Note that we don't support SQLite. We only support Postgres and MySQL (and we don't use the latter anyway).