feat(receiver): add HTTP metrics receiver with SQLite storage

Add a new receiver application under cmd/receiver that accepts metrics
via HTTP POST and stores them in SQLite using GORM. The receiver expects
GitHub Actions style execution context (org, repo, workflow, job, run_id).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/receiver/main.go

@@ -0,0 +1,77 @@
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"edp.buildth.ing/DevFW-CICD/forgejo-runner-resource-collector/internal/receiver"
+)
+
+const (
+	defaultAddr   = ":8080"
+	defaultDBPath = "metrics.db"
+)
+
+func main() {
+	addr := flag.String("addr", defaultAddr, "HTTP listen address")
+	dbPath := flag.String("db", defaultDBPath, "SQLite database path")
+	flag.Parse()
+
+	logger := slog.New(slog.NewJSONHandler(os.Stderr, &slog.HandlerOptions{
+		Level: slog.LevelInfo,
+	}))
+
+	store, err := receiver.NewStore(*dbPath)
+	if err != nil {
+		logger.Error("failed to open database", slog.String("error", err.Error()))
+		os.Exit(1)
+	}
+	defer func() { _ = store.Close() }()
+
+	handler := receiver.NewHandler(store, logger)
+	mux := http.NewServeMux()
+	handler.RegisterRoutes(mux)
+
+	server := &http.Server{
+		Addr:         *addr,
+		Handler:      mux,
+		ReadTimeout:  10 * time.Second,
+		WriteTimeout: 10 * time.Second,
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	sigChan := make(chan os.Signal, 1)
+	signal.Notify(sigChan, syscall.SIGINT, syscall.SIGTERM)
+
+	go func() {
+		sig := <-sigChan
+		logger.Info("received signal, shutting down", slog.String("signal", sig.String()))
+		cancel()
+
+		shutdownCtx, shutdownCancel := context.WithTimeout(context.Background(), 5*time.Second)
+		defer shutdownCancel()
+		_ = server.Shutdown(shutdownCtx)
+	}()
+
+	logger.Info("starting metrics receiver",
+		slog.String("addr", *addr),
+		slog.String("db", *dbPath),
+	)
+
+	if err := server.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+		fmt.Fprintf(os.Stderr, "error: %v\n", err)
+		os.Exit(1)
+	}
+
+	<-ctx.Done()
+	logger.Info("receiver stopped gracefully")
+}