feat(analyzer): add attribution fields to queue windows

[bryangingechen]

A new QueueWindowEventType TextChoices enum:

Value	FK column populated	Notes
REQUIRED_LABEL_ADDED	timeline_event	A required label was added
REQUIRED_LABEL_REMOVED	timeline_event	A required label was removed
FORBIDDEN_LABEL_ADDED	timeline_event	A forbidden label was added
FORBIDDEN_LABEL_REMOVED	timeline_event	A forbidden label was removed
CI_PASSED	check_run or status_context	CI state flipped to eligible
CI_FAILED	check_run or status_context	CI state flipped to ineligible
PR_OPENED	timeline_event	PR reopened (or opened fresh)
DRAFT_CONVERTED	timeline_event	PR converted from draft to ready
CONVERTED_TO_DRAFT	timeline_event	PR converted to draft
PR_CLOSED	timeline_event	PR closed or merged
HEAD_PUSHED	timeline_event	Force-push; resets CI state for new SHA
INITIAL_STATE	(all null)	PR was already eligible at window start
RULESET_EFFECTIVE	(all null)	Ruleset effective_from boundary reached
UNKNOWN	(all null)	Defensive fallback; should not appear in practice

At most one of the three FK columns is non-null for any open/close event; which one is set
is determined by event_type.

New Fields on PRQueueWindow

opened_by_event_type     = CharField(max_length=32, null=True, choices=QueueWindowEventType.choices)
opened_by_timeline_event = ForeignKey("syncer.PRTimelineEvent",   null=True, on_delete=SET_NULL, related_name="+")
opened_by_check_run      = ForeignKey("syncer.CommitCheckRun",     null=True, on_delete=SET_NULL, related_name="+")
opened_by_status_context = ForeignKey("syncer.CommitStatusContext",null=True, on_delete=SET_NULL, related_name="+")
opened_at_head_sha       = CharField(max_length=40, null=True)

closed_by_event_type     = CharField(max_length=32, null=True, choices=QueueWindowEventType.choices)
closed_by_timeline_event = ForeignKey("syncer.PRTimelineEvent",   null=True, on_delete=SET_NULL, related_name="+")
closed_by_check_run      = ForeignKey("syncer.CommitCheckRun",     null=True, on_delete=SET_NULL, related_name="+")
closed_by_status_context = ForeignKey("syncer.CommitStatusContext",null=True, on_delete=SET_NULL, related_name="+")
closed_at_head_sha       = CharField(max_length=40, null=True)

closed_by_* and closed_at_head_sha are null for windows that are still open (to_ts = None).

All FK columns use on_delete=SET_NULL. If a referenced row is deleted out-of-band the FK
silently becomes null; the rebuild-on-deletion mechanism (see below) ensures this state is
transient in practice.

prepared with claude code