feat(t-paliad-187): POST /api/tools/event-trigger handler + wiring

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.

cmd/server/main.go

@@ -154,6 +154,7 @@ func main() {
 				courts,
 				services.NewFristenrechnerService(rules, holidays, courts),
 			),
+			EventTrigger: services.NewEventTriggerService(pool, rules, holidays, courts),
 			Courts:         courts,
 			DeadlineSearch: services.NewDeadlineSearchService(pool),
 			EventCategory:  nil, // wired below; cross-link order matters