Rewrites generateDays to remap days positionally by day_number instead
of matching by date identity. Previously any date range shift with no
overlap would cascade-delete all day_assignments, day_notes, and
day_accommodations.
New behaviour:
- Shift/partial overlap: existing days remapped to new dates in order
- Shrink: overflow days become dateless (date=NULL) instead of deleted,
preserving all child data for manual reassignment
- Grow: existing days kept, new empty days appended
- Clear dates: all days nullified, content intact
Also fixes a UNIQUE(trip_id, day_number) collision that would occur when
spare dateless days remained after growing into a partially-dateless trip
(maxAssigned base was wrong).
Closes#646
jubnl
a438652a50