feat: implement configurable background jobs and toggle flags

Introduce environment variables to globally and individually control background jobs:
- `WARPBOX_JOBS_ENABLED` to toggle all background workers.
- `WARPBOX_CLEANUP_ENABLED` to toggle the expired box cleanup job.
- `WARPBOX_THUMBNAIL_ENABLED` to toggle the thumbnail generation job.

Refactor background tasks into a dedicated `backend/libs/jobs` package, allowing jobs to be registered, scheduled, and conditionally run based on the new configuration flags. Additionally, update the default maximum upload size in `.env.example` to 16GB and document the new settings in the README.

backend/libs/config/config.go

@@ -11,38 +11,44 @@ import (
 )
 
 type Config struct {
-	AppName        string
-	Environment    string
-	Addr           string
-	BaseURL        string
-	DataDir        string
-	AdminToken     string
-	StaticDir      string
-	TemplateDir    string
-	ReadTimeout    time.Duration
-	WriteTimeout   time.Duration
-	IdleTimeout    time.Duration
-	CleanupEvery   time.Duration
-	ThumbnailEvery time.Duration
-	MaxUploadSize  int64
+	AppName          string
+	Environment      string
+	Addr             string
+	BaseURL          string
+	DataDir          string
+	AdminToken       string
+	StaticDir        string
+	TemplateDir      string
+	ReadTimeout      time.Duration
+	WriteTimeout     time.Duration
+	IdleTimeout      time.Duration
+	JobsEnabled      bool
+	CleanupEnabled   bool
+	CleanupEvery     time.Duration
+	ThumbnailEnabled bool
+	ThumbnailEvery   time.Duration
+	MaxUploadSize    int64
 }
 
 func Load() (Config, error) {
 	cfg := Config{
-		AppName:        envString("WARPBOX_APP_NAME", "warpbox.dev"),
-		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")),
-		AdminToken:     envString("WARPBOX_ADMIN_TOKEN", ""),
-		StaticDir:      envString("WARPBOX_STATIC_DIR", defaultPath("static")),
-		TemplateDir:    envString("WARPBOX_TEMPLATE_DIR", defaultPath("templates")),
-		ReadTimeout:    envDuration("WARPBOX_READ_TIMEOUT", 15*time.Second),
-		WriteTimeout:   envDuration("WARPBOX_WRITE_TIMEOUT", 60*time.Second),
-		IdleTimeout:    envDuration("WARPBOX_IDLE_TIMEOUT", 120*time.Second),
-		CleanupEvery:   envDuration("WARPBOX_CLEANUP_EVERY", time.Hour),
-		ThumbnailEvery: envDuration("WARPBOX_THUMBNAIL_EVERY", time.Minute),
-		MaxUploadSize:  envMegabytes("WARPBOX_MAX_UPLOAD_SIZE_MB", 2048), // 2 GiB default.
+		AppName:          envString("WARPBOX_APP_NAME", "warpbox.dev"),
+		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")),
+		AdminToken:       envString("WARPBOX_ADMIN_TOKEN", ""),
+		StaticDir:        envString("WARPBOX_STATIC_DIR", defaultPath("static")),
+		TemplateDir:      envString("WARPBOX_TEMPLATE_DIR", defaultPath("templates")),
+		ReadTimeout:      envDuration("WARPBOX_READ_TIMEOUT", 15*time.Second),
+		WriteTimeout:     envDuration("WARPBOX_WRITE_TIMEOUT", 60*time.Second),
+		IdleTimeout:      envDuration("WARPBOX_IDLE_TIMEOUT", 120*time.Second),
+		JobsEnabled:      envBool("WARPBOX_JOBS_ENABLED", true),
+		CleanupEnabled:   envBool("WARPBOX_CLEANUP_ENABLED", true),
+		CleanupEvery:     envDuration("WARPBOX_CLEANUP_EVERY", time.Hour),
+		ThumbnailEnabled: envBool("WARPBOX_THUMBNAIL_ENABLED", true),
+		ThumbnailEvery:   envDuration("WARPBOX_THUMBNAIL_EVERY", time.Minute),
+		MaxUploadSize:    envMegabytes("WARPBOX_MAX_UPLOAD_SIZE_MB", 2048), // 2 GiB default.
 	}
 
 	if cfg.BaseURL == "" {
@@ -90,6 +96,19 @@ func envDuration(key string, fallback time.Duration) time.Duration {
 	return parsed
 }
 
+func envBool(key string, fallback bool) bool {
+	value := strings.TrimSpace(os.Getenv(key))
+	if value == "" {
+		return fallback
+	}
+
+	parsed, err := strconv.ParseBool(value)
+	if err != nil {
+		return fallback
+	}
+	return parsed
+}
+
 func envMegabytes(key string, fallback float64) int64 {
 	value := strings.TrimSpace(os.Getenv(key))
 	if value == "" {