Adds DecoTemplateController mirroring TemplateController: CRUD for deco
template groups, deco templates (flat list + by-id), item listing, and a
full-replace PUT for a deco template's item list.
Hardening (deviation from a literal port of the existing handlers, matching
the #144 S2 fix for ReplaceTemplateItemsHandler):
- CreateDecoTemplateHandler now validates DecoTemplateGroupId exists before
insert (previously a bad id hit the FK constraint at SaveChanges and
surfaced as a 500; now a 422).
- ReplaceDecoTemplateItemsHandler now rejects invalid items (unknown DecoId,
StartTime >= EndTime unless EndTime is the end-of-day sentinel 00:00:00,
or overlapping ranges) with a 422 instead of silently dropping/persisting
them - the same silent-drop/silent-overlap bug class already fixed for
templates.
Response DTOs serialize the raw item TimeSpans (via .TimeOfDay), so an
end-of-day item still round-trips as StartTime=22:00:00/EndTime=00:00:00
regardless of the ViewModel's day-wrapping DateTime representation.
