Return 400 instead of 404 for a malformed date query parameter (fixes #159)

Problem

GET /api/channels/{channelId}/messages (and the other message endpoints) take
startDate/endDate as @QueryParam Calendar, converted by
CalendarParamConverterProvider. On a malformed date, fromString throws
ProcessingException, which Jersey surfaces as 404 Not Found:

GET /api/channels/{id}/messages?startDate=1985   ->   404 { "message": "Not Found" }

404 is misleading for a bad parameter value — the caller should get a 400 that says
what was wrong.

Fix

Throw BadRequestException (400) from the converter, with a message naming the value and
the expected format:

} catch (DateTimeParseException e) {
    throw new BadRequestException("Invalid date '" + value + "'. Expected format yyyy-MM-dd'T'HH:mm:ss.SSSZ, for example 1985-10-26T09:00:00.000-0700.", e);
}

JAX-RS honors a WebApplicationException thrown from a ParamConverter (and no
ExceptionMappers are registered that would override it), so the response status is 400.
Valid dates are unaffected.

Tests

CalendarParamConverterProviderTest:

• null returns null
• a valid date round-trips (fromString then toString)
• a malformed date throws BadRequestException with status 400

./gradlew :server:test --tests "*CalendarParamConverterProviderTest*" → 3 tests, 0 failures.

Note on verification

The project has no Jersey HTTP test harness, so this is verified at the converter level —
the same approach as the existing MetaDataSearchParamConverterProviderTest. The
HTTP-level 400 follows from JAX-RS WebApplicationException handling as described above.