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/config/config.go

@@ -15,6 +15,7 @@ type Config struct {
 	Environment   string
 	Addr          string
 	BaseURL       string
+	DataDir       string
 	StaticDir     string
 	TemplateDir   string
 	ReadTimeout   time.Duration
@@ -29,6 +30,7 @@ func Load() (Config, error) {
 		Environment:   envString("WARPBOX_ENV", "development"),
 		Addr:          envString("WARPBOX_ADDR", ":8080"),
 		BaseURL:       strings.TrimRight(envString("WARPBOX_BASE_URL", "http://localhost:8080"), "/"),
+		DataDir:       envString("WARPBOX_DATA_DIR", defaultPath("data")),
 		StaticDir:     envString("WARPBOX_STATIC_DIR", defaultPath("static")),
 		TemplateDir:   envString("WARPBOX_TEMPLATE_DIR", defaultPath("templates")),
 		ReadTimeout:   envDuration("WARPBOX_READ_TIMEOUT", 15*time.Second),