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/router/router.go

@@ -5,6 +5,8 @@ import (
 	"net/http"
 
 	"mgit.msbls.de/m/KanzlAI-mGMT/internal/auth"
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/handlers"
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/services"
 
 	"github.com/jmoiron/sqlx"
 )
@@ -12,12 +14,34 @@ import (
 func New(db *sqlx.DB, authMW *auth.Middleware) http.Handler {
 	mux := http.NewServeMux()
 
+	// Services
+	caseSvc := services.NewCaseService(db)
+	partySvc := services.NewPartyService(db)
+
+	// Handlers
+	caseH := handlers.NewCaseHandler(caseSvc)
+	partyH := handlers.NewPartyHandler(partySvc)
+
 	// Public routes
 	mux.HandleFunc("GET /health", handleHealth(db))
 
 	// Authenticated API routes
 	api := http.NewServeMux()
-	api.HandleFunc("GET /api/cases", placeholder("cases"))
+
+	// Cases
+	api.HandleFunc("GET /api/cases", caseH.List)
+	api.HandleFunc("POST /api/cases", caseH.Create)
+	api.HandleFunc("GET /api/cases/{id}", caseH.Get)
+	api.HandleFunc("PUT /api/cases/{id}", caseH.Update)
+	api.HandleFunc("DELETE /api/cases/{id}", caseH.Delete)
+
+	// Parties (nested under cases for creation/listing, top-level for update/delete)
+	api.HandleFunc("GET /api/cases/{id}/parties", partyH.List)
+	api.HandleFunc("POST /api/cases/{id}/parties", partyH.Create)
+	api.HandleFunc("PUT /api/parties/{partyId}", partyH.Update)
+	api.HandleFunc("DELETE /api/parties/{partyId}", partyH.Delete)
+
+	// Placeholder routes for future phases
 	api.HandleFunc("GET /api/deadlines", placeholder("deadlines"))
 	api.HandleFunc("GET /api/appointments", placeholder("appointments"))
 	api.HandleFunc("GET /api/documents", placeholder("documents"))