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/handlers/upload.go

@@ -1,19 +1,49 @@
 package handlers
 
 import (
+	"errors"
 	"net/http"
+	"strconv"
 
 	"warpbox.dev/backend/libs/helpers"
+	"warpbox.dev/backend/libs/services"
 )
 
-type uploadPlaceholderResponse struct {
-	Message       string `json:"message"`
-	MaxUploadSize string `json:"maxUploadSize"`
+func (a *App) Upload(w http.ResponseWriter, r *http.Request) {
+	r.Body = http.MaxBytesReader(w, r.Body, a.uploadService.MaxUploadSize()*8)
+	if err := r.ParseMultipartForm(a.uploadService.MaxUploadSize() * 8); err != nil {
+		helpers.WriteJSONError(w, http.StatusBadRequest, "upload form could not be read")
+		return
+	}
+
+	files := r.MultipartForm.File["file"]
+	result, err := a.uploadService.CreateBox(files, services.UploadOptions{
+		MaxDays:      parseInt(r.FormValue("max_days")),
+		MaxDownloads: parseInt(r.FormValue("max_downloads")),
+	})
+	if err != nil {
+		a.logger.Warn("upload failed", "source", "user-upload", "code", 4001, "error", err.Error())
+		helpers.WriteJSONError(w, http.StatusBadRequest, err.Error())
+		return
+	}
+
+	helpers.WriteJSON(w, http.StatusCreated, result)
 }
 
-func (a *App) UploadPlaceholder(w http.ResponseWriter, r *http.Request) {
-	helpers.WriteJSON(w, http.StatusNotImplemented, uploadPlaceholderResponse{
-		Message:       "upload storage is not implemented yet; backend base is ready for the upload pipeline",
-		MaxUploadSize: a.uploadService.MaxUploadSizeLabel(),
-	})
+func parseInt(value string) int {
+	if value == "" {
+		return 0
+	}
+	parsed, err := strconv.Atoi(value)
+	if err != nil {
+		return 0
+	}
+	return parsed
+}
+
+func statusForDownloadError(err error) int {
+	if errors.Is(err, http.ErrMissingFile) {
+		return http.StatusNotFound
+	}
+	return http.StatusForbidden
 }