feat: add authentication and book upload

- Session-based login (username/password, 30-day cookie)
- SQLite user store with bcrypt password hashing (modernc.org/sqlite)
- Per-user upload permission (can_upload flag)
- Admin CLI (cmd/admin) for user management:
  user add/list/delete/set-upload
- Upload handler for EPUB/PDF with path-traversal protection
- All routes protected by requireAuth middleware;
  /upload additionally requires requireUpload
- Login/logout UI, upload form, logout button in header
- New env var: USERS_DB (default: users.db, gitignored)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

cmd/admin/main.go

@@ -0,0 +1,149 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+	"syscall"
+
+	"github.com/arnef/ebooks/internal/users"
+	"golang.org/x/term"
+)
+
+func main() {
+	if len(os.Args) < 3 {
+		usage()
+		os.Exit(1)
+	}
+
+	dbPath := getenv("USERS_DB", "users.db")
+	store, err := users.Open(dbPath)
+	if err != nil {
+		fatalf("open db: %v\n", err)
+	}
+	defer store.Close()
+
+	resource := os.Args[1]
+	action := os.Args[2]
+
+	switch resource {
+	case "user":
+		handleUser(store, action, os.Args[3:])
+	default:
+		usage()
+		os.Exit(1)
+	}
+}
+
+func handleUser(store *users.Store, action string, args []string) {
+	switch action {
+	case "add":
+		if len(args) < 1 {
+			fatalf("usage: user add <username> [--upload]\n")
+		}
+		username := args[0]
+		canUpload := len(args) > 1 && args[1] == "--upload"
+
+		password, err := readPassword(fmt.Sprintf("Passwort für '%s': ", username))
+		if err != nil {
+			fatalf("passwort lesen: %v\n", err)
+		}
+		confirm, err := readPassword("Passwort bestätigen: ")
+		if err != nil {
+			fatalf("passwort lesen: %v\n", err)
+		}
+		if password != confirm {
+			fatalf("Passwörter stimmen nicht überein.\n")
+		}
+
+		u, err := store.CreateUser(username, password, canUpload)
+		if err != nil {
+			fatalf("user anlegen: %v\n", err)
+		}
+		fmt.Printf("✓ Benutzer '%s' angelegt (upload: %s)\n", u.Username, yesNo(u.CanUpload))
+
+	case "list":
+		list, err := store.ListUsers()
+		if err != nil {
+			fatalf("user auflisten: %v\n", err)
+		}
+		if len(list) == 0 {
+			fmt.Println("Keine Benutzer vorhanden.")
+			return
+		}
+		fmt.Printf("%-20s  %s\n", "BENUTZERNAME", "UPLOAD")
+		fmt.Println(strings.Repeat("-", 32))
+		for _, u := range list {
+			fmt.Printf("%-20s  %s\n", u.Username, yesNo(u.CanUpload))
+		}
+
+	case "delete":
+		if len(args) < 1 {
+			fatalf("usage: user delete <username>\n")
+		}
+		if err := store.DeleteUser(args[0]); err != nil {
+			fatalf("user löschen: %v\n", err)
+		}
+		fmt.Printf("✓ Benutzer '%s' gelöscht.\n", args[0])
+
+	case "set-upload":
+		if len(args) < 2 {
+			fatalf("usage: user set-upload <username> <true|false>\n")
+		}
+		val, err := strconv.ParseBool(args[1])
+		if err != nil {
+			fatalf("ungültiger Wert '%s', erwartet true oder false\n", args[1])
+		}
+		if err := store.SetUpload(args[0], val); err != nil {
+			fatalf("upload-recht setzen: %v\n", err)
+		}
+		fmt.Printf("✓ Upload-Recht für '%s' auf %s gesetzt.\n", args[0], yesNo(val))
+
+	default:
+		usage()
+		os.Exit(1)
+	}
+}
+
+func readPassword(prompt string) (string, error) {
+	fmt.Print(prompt)
+	b, err := term.ReadPassword(int(syscall.Stdin))
+	fmt.Println()
+	if err != nil {
+		return "", err
+	}
+	return string(b), nil
+}
+
+func yesNo(b bool) string {
+	if b {
+		return "ja"
+	}
+	return "nein"
+}
+
+func getenv(key, fallback string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return fallback
+}
+
+func fatalf(format string, args ...any) {
+	fmt.Fprintf(os.Stderr, format, args...)
+	os.Exit(1)
+}
+
+func usage() {
+	fmt.Fprintln(os.Stderr, `ebooks-admin – Benutzerverwaltung
+
+Befehle:
+  user add <name> [--upload]          Benutzer anlegen (optional: Upload-Recht)
+  user list                           Alle Benutzer auflisten
+  user delete <name>                  Benutzer löschen
+  user set-upload <name> <true|false> Upload-Recht ändern
+
+Umgebungsvariablen:
+  USERS_DB  Pfad zur SQLite-Datenbank (Standard: users.db)`)
+}