Phase 3 Slice 6 handler. Decodes JSON body (eventTypeId, conceptId,
triggerDate, flags, courtId, perspective), validates required
fields (triggerDate + at least one identifier), parses UUIDs (400
on malformed), delegates to EventTriggerService.Trigger, surfaces
ErrInvalidInput as 400 with the service's German user-facing
message.
Wiring:
- dbServices gains an eventTrigger pointer (handlers package
internal type) wired from handlers.Services.EventTrigger.
- handlers.Services.EventTrigger is the new exported field; the
bundle constructor in main.go fills it from
NewEventTriggerService(pool, rules, holidays, courts).
- Route registered as POST /api/tools/event-trigger on the
protected mux, sibling to the existing /api/tools/fristenrechner
and /api/tools/event-deadlines endpoints.
Returns 503 when DATABASE_URL is unset (matches every other
calculator endpoint's behaviour). Returns same JSON shape as
/api/tools/fristenrechner so the frontend can render with the
existing timeline renderer.