feat: add errors, throughput, and billing sections to stats dashboard

- GET /api/stats?window=7d: pre-aggregated SQL queries for errors, throughput, billing
- Errors section: category summary (quota/rate_limit/timeout/git/failed) + failure table
- Throughput section: stacked hourly bar chart (completed/failed/other) over 7d
- Billing section: KPIs (7d total, avg/day, cost/run) + daily cost bar chart

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

2 files changed, 18 insertions, 0 deletions

diff --git a/internal/api/executions.go b/internal/api/executions.go
index 29af139..4d8ba9c 100644
--- a/internal/api/executions.go
+++ b/internal/api/executions.go
@@ -86,6 +86,23 @@ func (s *Server) handleGetExecutionLog(w http.ResponseWriter, r *http.Request) {
 	fmt.Fprint(w, content)
 }
 
+// handleGetDashboardStats returns pre-aggregated error, throughput, and billing stats.
+// GET /api/stats?window=7d|24h
+func (s *Server) handleGetDashboardStats(w http.ResponseWriter, r *http.Request) {
+	window := 7 * 24 * time.Hour
+	if r.URL.Query().Get("window") == "24h" {
+		window = 24 * time.Hour
+	}
+	since := time.Now().Add(-window)
+
+	stats, err := s.store.QueryDashboardStats(since)
+	if err != nil {
+		writeJSON(w, http.StatusInternalServerError, map[string]string{"error": err.Error()})
+		return
+	}
+	writeJSON(w, http.StatusOK, stats)
+}
+
 // handleGetAgentStatus returns the current status of all agents and recent rate-limit events.
 // GET /api/agents/status?since=<RFC3339>
 func (s *Server) handleGetAgentStatus(w http.ResponseWriter, r *http.Request) {
diff --git a/internal/api/server.go b/internal/api/server.go
index 2d5c308..0127ab9 100644
--- a/internal/api/server.go
+++ b/internal/api/server.go
@@ -121,6 +121,7 @@ func (s *Server) routes() {
 	s.mux.HandleFunc("GET /api/tasks/{id}/subtasks", s.handleListSubtasks)
 	s.mux.HandleFunc("GET /api/tasks/{id}/executions", s.handleListExecutions)
 	s.mux.HandleFunc("GET /api/executions", s.handleListRecentExecutions)
+	s.mux.HandleFunc("GET /api/stats", s.handleGetDashboardStats)
 	s.mux.HandleFunc("GET /api/agents/status", s.handleGetAgentStatus)
 	s.mux.HandleFunc("GET /api/executions/{id}", s.handleGetExecution)
 	s.mux.HandleFunc("GET /api/executions/{id}/log", s.handleGetExecutionLog)