feat: add admin console, cleanup, and thumbnail workers

- Implement a token-authenticated admin console at `/admin` with overview metrics and file management.
- Add a background worker to periodically clean up expired boxes based on `WARPBOX_CLEANUP_EVERY`.
- Add a background worker to generate image and video thumbnails based on `WARPBOX_THUMBNAIL_EVERY`.
- Update file storage paths to use `@each@` and `@thumb@` prefixes to separate original files from thumbnails.
- Add severity fields to startup logs and update configuration templates.

backend/libs/httpserver/server.go

@@ -3,6 +3,7 @@ package httpserver
 import (
 	"log/slog"
 	"net/http"
+	"time"
 
 	"warpbox.dev/backend/libs/config"
 	"warpbox.dev/backend/libs/handlers"
@@ -21,6 +22,8 @@ func New(cfg config.Config, logger *slog.Logger) (*http.Server, error) {
 	if err != nil {
 		return nil, err
 	}
+	stopCleanup := startCleanup(uploadService, cfg.CleanupEvery, logger)
+	stopThumbnails := startThumbnails(uploadService, cfg.ThumbnailEvery, logger)
 	app := handlers.NewApp(cfg, logger, renderer, uploadService)
 
 	router := http.NewServeMux()
@@ -43,10 +46,81 @@ func New(cfg config.Config, logger *slog.Logger) (*http.Server, error) {
 		IdleTimeout:  cfg.IdleTimeout,
 	}
 	server.RegisterOnShutdown(func() {
+		stopCleanup()
+		stopThumbnails()
 		if err := uploadService.Close(); err != nil {
-			logger.Error("failed to close upload service", "source", "shutdown", "error", err.Error())
+			logger.Error("failed to close upload service", "source", "shutdown", "severity", "error", "error", err.Error())
 		}
 	})
 
 	return server, nil
 }
+
+func startCleanup(uploadService *services.UploadService, interval time.Duration, logger *slog.Logger) func() {
+	if interval <= 0 {
+		return func() {}
+	}
+
+	stop := make(chan struct{})
+	go func() {
+		if cleaned, err := uploadService.CleanupExpired(); err != nil {
+			logger.Warn("initial cleanup failed", "source", "housekeeping", "severity", "warn", "code", 4201, "error", err.Error())
+		} else if cleaned > 0 {
+			logger.Info("initial cleanup complete", "source", "housekeeping", "severity", "user_activity", "code", 2202, "cleaned", cleaned)
+		}
+
+		ticker := time.NewTicker(interval)
+		defer ticker.Stop()
+		for {
+			select {
+			case <-ticker.C:
+				if _, err := uploadService.CleanupExpired(); err != nil {
+					logger.Warn("scheduled cleanup failed", "source", "housekeeping", "severity", "warn", "code", 4202, "error", err.Error())
+				}
+			case <-stop:
+				return
+			}
+		}
+	}()
+
+	return func() {
+		close(stop)
+	}
+}
+
+func startThumbnails(uploadService *services.UploadService, interval time.Duration, logger *slog.Logger) func() {
+	if interval <= 0 {
+		return func() {}
+	}
+
+	stop := make(chan struct{})
+	run := func(source string) {
+		result, err := uploadService.GenerateMissingThumbnails()
+		if err != nil {
+			logger.Warn("thumbnail job failed", "source", "thumbnail", "severity", "warn", "code", 4203, "error", err.Error())
+			return
+		}
+		if result.Generated > 0 || result.Failed > 0 {
+			logger.Info("thumbnail job run", "source", source, "severity", "user_activity", "code", 2204, "generated", result.Generated, "failed", result.Failed)
+		}
+	}
+
+	go func() {
+		run("thumbnail")
+
+		ticker := time.NewTicker(interval)
+		defer ticker.Stop()
+		for {
+			select {
+			case <-ticker.C:
+				run("thumbnail")
+			case <-stop:
+				return
+			}
+		}
+	}()
+
+	return func() {
+		close(stop)
+	}
+}