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/libs/logging/logger.go

@@ -0,0 +1,105 @@
+package logging
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"log/slog"
+	"os"
+	"path/filepath"
+	"sync"
+	"time"
+)
+
+type Closer func() error
+
+type jsonLineHandler struct {
+	mu    *sync.Mutex
+	out   io.Writer
+	attrs []slog.Attr
+}
+
+func New(dataDir string) (*slog.Logger, Closer, error) {
+	logDir := filepath.Join(dataDir, "logs")
+	if err := os.MkdirAll(logDir, 0o755); err != nil {
+		return nil, nil, err
+	}
+
+	path := filepath.Join(logDir, time.Now().Format("2006-01-02")+".log")
+	file, err := os.OpenFile(path, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0o644)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	handler := &jsonLineHandler{
+		mu:  &sync.Mutex{},
+		out: io.MultiWriter(os.Stdout, file),
+	}
+
+	return slog.New(handler), file.Close, nil
+}
+
+func (h *jsonLineHandler) Enabled(context.Context, slog.Level) bool {
+	return true
+}
+
+func (h *jsonLineHandler) Handle(_ context.Context, record slog.Record) error {
+	entry := map[string]any{
+		"date":     record.Time.Format("2006-01-02"),
+		"time":     record.Time.Format("15:04:05"),
+		"source":   "app",
+		"code":     0,
+		"severity": severity(record.Level),
+		"log":      record.Message,
+	}
+
+	for _, attr := range h.attrs {
+		applyAttr(entry, attr)
+	}
+	record.Attrs(func(attr slog.Attr) bool {
+		applyAttr(entry, attr)
+		return true
+	})
+
+	line, err := json.Marshal(entry)
+	if err != nil {
+		return err
+	}
+
+	h.mu.Lock()
+	defer h.mu.Unlock()
+	_, err = h.out.Write(append(line, '\n'))
+	return err
+}
+
+func (h *jsonLineHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
+	next := &jsonLineHandler{
+		mu:    h.mu,
+		out:   h.out,
+		attrs: append([]slog.Attr{}, h.attrs...),
+	}
+	next.attrs = append(next.attrs, attrs...)
+	return next
+}
+
+func (h *jsonLineHandler) WithGroup(string) slog.Handler {
+	return h
+}
+
+func applyAttr(entry map[string]any, attr slog.Attr) {
+	attr.Value = attr.Value.Resolve()
+	entry[attr.Key] = attr.Value.Any()
+}
+
+func severity(level slog.Level) string {
+	switch {
+	case level >= slog.LevelError:
+		return "high"
+	case level >= slog.LevelWarn:
+		return "medium"
+	case level <= slog.LevelDebug:
+		return "low"
+	default:
+		return "info"
+	}
+}