feat(t-paliad-110): add EventService + /api/events + /api/events/summary

PR-1 of the Fristen+Termine unification (t-paliad-110). Backend layer
only — no frontend changes; the existing /deadlines and /appointments
pages still render the type-specific UIs.

EventService delegates to DeadlineService + AppointmentService for the
actual reads (no duplicate visibility logic, no duplicate event_type
hydration), then projects both into the discriminated EventListItem
union and merges/sorts by event_date asc. The handler exposes:

  GET /api/events?type=deadline|appointment|all&status=…&project_id=…
                  &event_type=…&type_filter=…&from=…&to=…
  GET /api/events/summary?type=…&project_id=…

Bucket model (per t-paliad-110 spec, supersedes t-106):
- four universal cards: Heute · Diese Woche · Nächste Woche · Später
- Überfällig is deadline-only, conditional, alarm-styled when > 0
- Erledigt drops from the card row; stays available as a filter option
- appointments have no completed_at — past appointments aren't bucketed

The deadline-side cutoffs reuse computeDeadlineBucketBounds so
/api/events/summary and /api/deadlines/summary can never disagree.

Existing /api/deadlines and /api/appointments stay untouched —
calendars, project-detail panes, and CalDAV consumers still call them
directly.

cmd/server/main.go

@@ -119,12 +119,13 @@ func main() {
 		mailSvc.SetTemplateService(emailTemplateSvc)
 
 		eventTypeSvc := services.NewEventTypeService(pool, users)
+		deadlineSvc := services.NewDeadlineService(pool, projectSvc, eventTypeSvc)
 		svcBundle = &handlers.Services{
 			Project:        projectSvc,
 			Team:           teamSvc,
 			PartnerUnit:    partnerUnitSvc,
 			Party:          services.NewPartyService(pool, projectSvc),
-			Deadline:       services.NewDeadlineService(pool, projectSvc, eventTypeSvc),
+			Deadline:       deadlineSvc,
 			Appointment:    appointmentSvc,
 			CalDAV:         caldavSvc,
 			Rules:          rules,
@@ -142,6 +143,7 @@ func main() {
 			Audit:          services.NewAuditService(pool),
 			EmailTemplate:  emailTemplateSvc,
 			Link:           services.NewLinkService(pool),
+			Event:          services.NewEventService(pool, deadlineSvc, appointmentSvc),
 		}
 		log.Println("Phase B services initialised")