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/nidusctl/main.go

@@ -61,6 +61,8 @@ func run(args []string) int {
 		return runCalendar(dbase, st, rest[1:])
 	case "addressbook", "card":
 		return runAddressBook(dbase, st, rest[1:])
+	case "migrate":
+		return runMigrate(st, rest[1:])
 	case "help", "-h", "--help":
 		usage()
 		return 0
@@ -88,20 +90,23 @@ Usage:
   nidusctl [-config config.yaml] calendar unshare  <owner> <calendar> <user>
   nidusctl [-config config.yaml] calendar shares   <owner> <calendar>
 
-  nidusctl [-config config.yaml] addressbook create  <owner> <book>
-  nidusctl [-config config.yaml] addressbook delete  <owner> <book>
-  nidusctl [-config config.yaml] addressbook list    <owner>
-  nidusctl [-config config.yaml] addressbook share   <owner> <book> <user> <read|write>
-  nidusctl [-config config.yaml] addressbook unshare <owner> <book> <user>
-  nidusctl [-config config.yaml] addressbook shares  <owner> <book>
+   nidusctl [-config config.yaml] addressbook create  <owner> <book>
+   nidusctl [-config config.yaml] addressbook delete  <owner> <book>
+   nidusctl [-config config.yaml] addressbook list    <owner>
+   nidusctl [-config config.yaml] addressbook share   <owner> <book> <user> <read|write>
+   nidusctl [-config config.yaml] addressbook unshare <owner> <book> <user>
+   nidusctl [-config config.yaml] addressbook shares  <owner> <book>
+
+   nidusctl [-config config.yaml] migrate [--verbose]
 
 Examples:
   nidusctl user create alice --display-name "Alice Smith" --email alice@example.com
   nidusctl calendar create alice work
-  nidusctl calendar share alice work bob write
-  nidusctl calendar shares alice work
-  nidusctl calendar unshare alice work bob
-`)
+   nidusctl calendar share alice work bob write
+   nidusctl calendar shares alice work
+   nidusctl calendar unshare alice work bob
+   nidusctl migrate
+ `)
 }
 
 // newFlagSet creates a flag.FlagSet configured for subcommand parsing
@@ -359,6 +364,20 @@ func runAddressBook(dbase *db.DB, st *store.Store, args []string) int {
 	}
 }
 
+func runMigrate(st *store.Store, args []string) int {
+	fs := newFlagSet("migrate")
+	if err := fs.Parse(args); err != nil {
+		return 2
+	}
+
+	if err := st.Migrate(); err != nil {
+		fmt.Fprintf(os.Stderr, "migration failed: %v\n", err)
+		return 1
+	}
+	fmt.Println("Migration completed successfully")
+	return 0
+}
+
 // warnIfUnknownUser reports (without failing) if username is not a
 // registered user — the share is still recorded, since a user could be
 // created afterwards.