feat(v3): add v3 stack — backend rewrite, renamed env vars, docs

- New Go backend binary (backend + runner) replacing old scraper/
- Rename SCRAPER_API_URL → BACKEND_API_URL in UI env and docker-compose
- Rename scraperFetch → backendFetch across all 19 UI server files
- Remove SCRAPER_PROXY env var and proxy transport from browser.Config
- Add Meilisearch, Valkey, Caddy to docker-compose
- Add docs/: api-endpoints.md, request-flow.mermaid.md, data-flow.mermaid.md

v3/backend/internal/runner/metrics.go

@@ -0,0 +1,92 @@
+package runner
+
+// metrics.go — lightweight HTTP metrics endpoint for the runner.
+//
+// GET /metrics returns a JSON document with live task counters and uptime.
+// No external dependency (no Prometheus); plain net/http only.
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"log/slog"
+	"net"
+	"net/http"
+	"time"
+)
+
+// metricsServer serves GET /metrics for the runner process.
+type metricsServer struct {
+	addr string
+	r    *Runner
+	log  *slog.Logger
+}
+
+func newMetricsServer(addr string, r *Runner, log *slog.Logger) *metricsServer {
+	return &metricsServer{addr: addr, r: r, log: log}
+}
+
+// ListenAndServe starts the HTTP server and blocks until ctx is cancelled or
+// a fatal listen error occurs.
+func (ms *metricsServer) ListenAndServe(ctx context.Context) error {
+	mux := http.NewServeMux()
+	mux.HandleFunc("GET /metrics", ms.handleMetrics)
+	mux.HandleFunc("GET /health", ms.handleHealth)
+
+	srv := &http.Server{
+		Addr:         ms.addr,
+		Handler:      mux,
+		ReadTimeout:  5 * time.Second,
+		WriteTimeout: 5 * time.Second,
+		BaseContext:  func(_ net.Listener) context.Context { return ctx },
+	}
+
+	errCh := make(chan error, 1)
+	go func() {
+		ms.log.Info("runner: metrics server listening", "addr", ms.addr)
+		errCh <- srv.ListenAndServe()
+	}()
+
+	select {
+	case <-ctx.Done():
+		shutCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+		defer cancel()
+		_ = srv.Shutdown(shutCtx)
+		return nil
+	case err := <-errCh:
+		return fmt.Errorf("runner: metrics server: %w", err)
+	}
+}
+
+// handleMetrics handles GET /metrics.
+// Response shape (JSON):
+//
+//	{
+//	  "tasks_running":   N,
+//	  "tasks_completed": N,
+//	  "tasks_failed":    N,
+//	  "uptime_seconds":  N
+//	}
+func (ms *metricsServer) handleMetrics(w http.ResponseWriter, _ *http.Request) {
+	uptimeSec := int64(time.Since(ms.r.startedAt).Seconds())
+	metricsWriteJSON(w, 0, map[string]int64{
+		"tasks_running":   ms.r.tasksRunning.Load(),
+		"tasks_completed": ms.r.tasksCompleted.Load(),
+		"tasks_failed":    ms.r.tasksFailed.Load(),
+		"uptime_seconds":  uptimeSec,
+	})
+}
+
+// handleHealth handles GET /health — simple liveness probe for the metrics server.
+func (ms *metricsServer) handleHealth(w http.ResponseWriter, _ *http.Request) {
+	metricsWriteJSON(w, 0, map[string]string{"status": "ok"})
+}
+
+// metricsWriteJSON writes v as a JSON response with the given status code.
+func metricsWriteJSON(w http.ResponseWriter, status int, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	if status != 0 {
+		w.WriteHeader(status)
+	}
+	_ = json.NewEncoder(w).Encode(v)
+}