feat: add dashboard aggregation endpoint (Phase 2I)

GET /api/dashboard returns aggregated data:
- deadline_summary: overdue, due this/next week, ok counts
- case_summary: active, new this month, closed counts
- upcoming_deadlines: next 7 days with case info
- upcoming_appointments: next 7 days
- recent_activity: last 10 case events

Uses efficient CTE query for summaries. Also fixes duplicate
writeJSON/writeError declarations in appointments handler.

backend/internal/router/router.go

@@ -27,6 +27,8 @@ func New(db *sqlx.DB, authMW *auth.Middleware) http.Handler {
 	// Middleware
 	tenantResolver := auth.NewTenantResolver(tenantSvc)
 
+	dashboardSvc := services.NewDashboardService(db)
+
 	// Handlers
 	tenantH := handlers.NewTenantHandler(tenantSvc)
 	caseH := handlers.NewCaseHandler(caseSvc)
@@ -35,6 +37,7 @@ func New(db *sqlx.DB, authMW *auth.Middleware) http.Handler {
 	deadlineH := handlers.NewDeadlineHandlers(deadlineSvc, db)
 	ruleH := handlers.NewDeadlineRuleHandlers(deadlineRuleSvc)
 	calcH := handlers.NewCalculateHandlers(calculator, deadlineRuleSvc)
+	dashboardH := handlers.NewDashboardHandler(dashboardSvc)
 
 	// Public routes
 	mux.HandleFunc("GET /health", handleHealth(db))
@@ -86,6 +89,9 @@ func New(db *sqlx.DB, authMW *auth.Middleware) http.Handler {
 	scoped.HandleFunc("PUT /api/appointments/{id}", apptH.Update)
 	scoped.HandleFunc("DELETE /api/appointments/{id}", apptH.Delete)
 
+	// Dashboard
+	scoped.HandleFunc("GET /api/dashboard", dashboardH.Get)
+
 	// Placeholder routes for future phases
 	scoped.HandleFunc("GET /api/documents", placeholder("documents"))