feat: time tracking + billing — hourly rates, time entries, invoices (P1)

Database: time_entries, billing_rates, invoices tables with RLS.
Backend: CRUD services+handlers for time entries, billing rates, invoices.
  - Time entries: list/create/update/delete, summary by case/user/month
  - Billing rates: upsert with auto-close previous, current rate lookup
  - Invoices: create with auto-number (RE-YYYY-NNN), status transitions
    (draft->sent->paid, cancellation), link time entries on invoice create
API: 11 new endpoints under /api/time-entries, /api/billing-rates, /api/invoices
Frontend: Zeiterfassung tab on case detail, /abrechnung overview with filters,
  /abrechnung/rechnungen list+detail with status actions, billing rates settings
Also: resolved merge conflicts between audit-trail and role-based branches,
  added missing types (Notification, AuditLogResponse, NotificationPreferences)

backend/internal/handlers/deadlines.go

@@ -198,18 +198,9 @@ func (h *DeadlineHandlers) Delete(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-<<<<<<< HEAD
-	if err := h.deadlines.Delete(tenantID, deadlineID); err != nil {
-		writeError(w, http.StatusNotFound, "deadline not found")
-||||||| 82878df
-	err = h.deadlines.Delete(tenantID, deadlineID)
-	if err != nil {
-		writeError(w, http.StatusNotFound, err.Error())
-=======
 	err = h.deadlines.Delete(r.Context(), tenantID, deadlineID)
 	if err != nil {
 		writeError(w, http.StatusNotFound, err.Error())
->>>>>>> mai/knuth/p0-audit-trail-append
 		return
 	}