feat: add configurable data directory and file-based logging

Introduce the `WARPBOX_DATA_DIR` environment variable to define where runtime data is stored. This directory will house uploaded files, the bbolt metadata database, and application logs.

Changes include:
- Added `WARPBOX_DATA_DIR` to configuration, defaulting to `./data`.
- Implemented a custom logging package that writes JSONL logs to the data directory.
- Updated `.gitignore` and `.env.example` to support the new data directory.
- Documented the runtime data structure in `README.md`.
- Updated the frontend upload script to handle form submission and display results.

backend/cmd/warpbox/main.go

@@ -3,7 +3,6 @@ package main
 import (
 	"context"
 	"errors"
-	"log/slog"
 	"net/http"
 	"os"
 	"os/signal"
@@ -12,28 +11,32 @@ import (
 
 	"warpbox.dev/backend/libs/config"
 	"warpbox.dev/backend/libs/httpserver"
+	"warpbox.dev/backend/libs/logging"
 )
 
 func main() {
-	logger := slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{
-		Level: slog.LevelInfo,
-	}))
-
 	cfg, err := config.Load()
 	if err != nil {
-		logger.Error("failed to load config", "error", err)
+		os.Stderr.WriteString("failed to load config: " + err.Error() + "\n")
 		os.Exit(1)
 	}
 
+	logger, closeLogs, err := logging.New(cfg.DataDir)
+	if err != nil {
+		os.Stderr.WriteString("failed to create logger: " + err.Error() + "\n")
+		os.Exit(1)
+	}
+	defer closeLogs()
+
 	server, err := httpserver.New(cfg, logger)
 	if err != nil {
-		logger.Error("failed to create server", "error", err)
+		logger.Error("failed to create server", "source", "startup", "error", err.Error())
 		os.Exit(1)
 	}
 
 	errs := make(chan error, 1)
 	go func() {
-		logger.Info("warpbox server starting", "addr", cfg.Addr, "env", cfg.Environment)
+		logger.Info("warpbox server starting", "source", "startup", "addr", cfg.Addr, "env", cfg.Environment)
 		errs <- server.ListenAndServe()
 	}()
 
@@ -43,18 +46,18 @@ func main() {
 	select {
 	case err := <-errs:
 		if err != nil && !errors.Is(err, http.ErrServerClosed) {
-			logger.Error("server stopped unexpectedly", "error", err)
+			logger.Error("server stopped unexpectedly", "source", "startup", "error", err.Error())
 			os.Exit(1)
 		}
 	case sig := <-shutdown:
-		logger.Info("shutdown signal received", "signal", sig.String())
+		logger.Info("shutdown signal received", "source", "startup", "signal", sig.String())
 		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 		defer cancel()
 
 		if err := server.Shutdown(ctx); err != nil {
-			logger.Error("graceful shutdown failed", "error", err)
+			logger.Error("graceful shutdown failed", "source", "startup", "error", err.Error())
 			os.Exit(1)
 		}
-		logger.Info("server stopped")
+		logger.Info("server stopped", "source", "startup")
 	}
 }