refactor: unify data directory structure

Unify user data directory from fragmented layout to consistent nested format:

- Move WebDAV from: data/files/<username> → data/<username>/files/
- Move CalDAV from: data/<username>/cal-<name> → data/<username>/calendars/<name>
- Move CardDAV from: data/<username>/card-<name> → data/<username>/addressbooks/<name>

Changes:
- internal/store/store.go: Update collectionPath() to map collection names
- internal/store/migrate.go: Add idempotent Migrate() method
- internal/store/migrate_test.go: Comprehensive migration tests
- internal/webdav/handler.go: Use new unified path structure
- cmd/server/main.go: Auto-run migration on startup
- tools/nidusctl/main.go: Add migrate subcommand
- Update tests to verify new structure

URL endpoints unchanged - only on-disk structure modified. All tests pass.

tools/migrate/main.go

@@ -0,0 +1,63 @@
+// Command migrate is a tool to restructure the data directory from the
+// old format to the new unified format.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log/slog"
+	"os"
+
+	"github.com/yourusername/caldav-server/internal/config"
+	"github.com/yourusername/caldav-server/internal/store"
+)
+
+func main() {
+	os.Exit(run(os.Args[1:]))
+}
+
+func run(args []string) int {
+	fs := flag.NewFlagSet("migrate", flag.ContinueOnError)
+	cfgPath := fs.String("config", "config.yaml", "path to configuration file")
+	verbose := fs.Bool("verbose", false, "enable verbose output")
+	if err := fs.Parse(args); err != nil {
+		return 2
+	}
+
+	cfg, err := config.Load(*cfgPath)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "error loading config: %v\n", err)
+		return 1
+	}
+
+	logger := buildLogger(*verbose)
+
+	st, err := store.NewStore(cfg.Storage.DataDir)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "error opening store %q: %v\n", cfg.Storage.DataDir, err)
+		return 1
+	}
+
+	logger.Info("starting migration", "data_dir", cfg.Storage.DataDir)
+
+	if err := st.Migrate(); err != nil {
+		logger.Error("migration failed", "error", err)
+		fmt.Fprintf(os.Stderr, "migration failed: %v\n", err)
+		return 1
+	}
+
+	logger.Info("migration completed successfully")
+	fmt.Println("Migration completed successfully")
+	return 0
+}
+
+func buildLogger(verbose bool) *slog.Logger {
+	level := slog.LevelInfo
+	if verbose {
+		level = slog.LevelDebug
+	}
+	handler := slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{
+		Level: level,
+	})
+	return slog.New(handler)
+}