Move pixad's startup from cmd/pixad into internal/app (closes #206)

cmd/pixad/main.go built the command line and its --config flag, set
PIXA_CONFIG_PATH from that flag, ignored SIGPIPE and started the fx
app. REPO_POLICIES.md now requires cmd/ to hold only one call into
internal/ or pkg/, so that code moves unchanged to Run in the new
internal/app package, and main calls app.Run(Version). Version stays
in main, so the -X main.Version build flags in the Dockerfile and the
Makefile do not change.

Model: opus-5-5

cmd/pixad/main.go

@@ -1,69 +1,10 @@
 // Package main is the entry point for the pixad image proxy server.
 package main
 
-import (
-	"fmt"
-	"os"
-	"os/signal"
-	"syscall"
-
-	"github.com/spf13/cobra"
-	"go.uber.org/fx"
-	"sneak.berlin/go/pixa/internal/config"
-	"sneak.berlin/go/pixa/internal/database"
-	"sneak.berlin/go/pixa/internal/globals"
-	"sneak.berlin/go/pixa/internal/handlers"
-	"sneak.berlin/go/pixa/internal/healthcheck"
-	"sneak.berlin/go/pixa/internal/logger"
-	"sneak.berlin/go/pixa/internal/middleware"
-	"sneak.berlin/go/pixa/internal/server"
-)
+import "sneak.berlin/go/pixa/internal/app"
 
 var Version string //nolint:gochecknoglobals // set by ldflags
 
-var configPath string //nolint:gochecknoglobals // cobra flag
-
 func main() {
-	rootCmd := &cobra.Command{
-		Use:   "pixad",
-		Short: "Pixa image caching proxy server",
-		Run:   run,
-	}
-
-	rootCmd.Flags().StringVarP(&configPath, "config", "c", "", "path to config file")
-
-	err := rootCmd.Execute()
-	if err != nil {
-		fmt.Fprintln(os.Stderr, err)
-		os.Exit(1)
-	}
-}
-
-func run(_ *cobra.Command, _ []string) {
-	globals.Version = Version
-
-	// Set config path in environment if specified via flag
-	if configPath != "" {
-		_ = os.Setenv("PIXA_CONFIG_PATH", configPath)
-	}
-
-	// A write to a closed stdout or stderr must not end the process.
-	signal.Ignore(syscall.SIGPIPE)
-
-	fx.New(
-		fx.Provide(
-			config.New,
-			database.New,
-			globals.New,
-			handlers.New,
-			logger.New,
-			server.New,
-			middleware.New,
-			healthcheck.New,
-		),
-		fx.Invoke(
-			func(log *logger.Logger) { log.Identify() },
-			func(*server.Server) {},
-		),
-	).Run()
+	app.Run(Version)
 }