Allow assigning talks and venues a priority

[Kunsi]

I'm trying to integrate this into https://github.com/seibert-media/pretalx-halfnarp, but slotmachine currently has no way of assigning priorities to talks or venues.

What halfnarp does is allow participants to vote on a unsorted list of talks that are supposed to happen, to allow organizers to schedule more interesting talks into bigger venues.

I would love to see support for this in slotmachine.

[russss]

We do the voting as well, but currently we just set preferred_venues to the single most suitable venue:

https://github.com/emfcamp/Website/blob/278792a372563035578fb01d87d9f578d257122f/apps/cfp/scheduler.py#L122
(this has not been updated for the new slotmachine interface yet)

It would be nice to be able to express this preference better though.

[Kunsi]

Proposed Interface:

talks = [
    Talk(
        # Talk ID for identifying talks in the result
        id=1,
        # Speaker IDs - the system will avoid scheduling two talks from the same speaker at the same time.
        speakers={1, 2},
        # Duration of the talk in minutes
        duration=30,
        # Time ranges when the talk is allowed to be scheduled
        allowed_times=[
            (datetime(2026, 5, 16, 12, 0, 0), datetime(2026, 5, 16, 19, 0, 0))
        ],
        # priority affects where talks will get scheduled - higher priority means bigger room and/or earlier slot (non-negative integer or Null)
        priority=5,
    ),
    ...
]

venues = [
    # this doesn't currently exist, we'd need to implement that
    Venue(
        # Venue ID for identifying rooms in the result
        id=1,
        # Time ranges when the venue allows content to be scheduled
        allowed_times=[
            (datetime(2026, 5, 16, 12, 0, 0), datetime(2026, 5, 16, 19, 0, 0))
        ],
        # capacity works the same as priority in talks (non-negative integer or Null)
        capacity=199,
    ),
    ...
]

problem = SchedulingProblem(
    talks=talks,
    venues=venues,
    # The length of a "slot" in minutes - this is the granularity the scheduler will operate at.
    # All durations and allowed times must be multiples of this number.
    slot_duration=10
)

I think the scheduler should put a higher-priority talk into a bigger venue, but if that's not possible due to availabilities the talk should get scheduled in the next-smaller venue (and so on) instead of in a random venue (which is the current behaviour if you set preferred_venues to a single item). Higher-priority talks should also get scheduled earlier in the day.