feat: add case + party CRUD with case events (Phase 1B)

- CaseService: list (paginated, filterable), get detail (with parties,
  events, deadline count), create, update, soft-delete (archive)
- PartyService: list by case, create, update, delete
- Auto-create case_events on case creation, status change, party add,
  and case archive
- Auth middleware now resolves tenant_id from user_tenants table
- All operations scoped to tenant_id from auth context

backend/internal/handlers/helpers.go

@@ -0,0 +1,16 @@
+package handlers
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+func writeJSON(w http.ResponseWriter, status int, v interface{}) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	json.NewEncoder(w).Encode(v)
+}
+
+func writeError(w http.ResponseWriter, status int, message string) {
+	writeJSON(w, status, map[string]string{"error": message})
+}