Move users, calendars, and address books from config.yaml into the database

BREAKING CHANGE: the users:/config-based collection setup is gone. All
user, calendar, and address-book data now lives in the SQLite DB
(internal/db) and is managed exclusively via nidusctl or the web UI.
Existing deployments must recreate their users after upgrading:
  nidusctl user create <username>
  nidusctl calendar create <username> <name>
  nidusctl addressbook create <username> <name>

- internal/db: new users, calendars, addressbooks tables with FK cascade
  delete; foreign_keys pragma enabled; internal/db/users.go implements
  full CRUD + bcrypt auth (CreateUser, VerifyPassword, ListUsers,
  CreateCalendar/AddressBook, etc).
- internal/config: removed Users/UserConfig entirely.
- internal/auth: Basic Auth now checks credentials via db.DB instead of
  cfg.Users.
- internal/caldav, internal/carddav: ListCalendars/ListAddressBooks and
  Create/Delete now backed by the DB.
- internal/web: login uses db.VerifyPassword; new resources.go adds
  create/delete handlers for calendars/address books at
  /web/resources/{calendar,addressbook}; dashboard gained create forms
  and per-card delete buttons (templ + htmx, no hyperscript).
- tools/nidusctl: new user create/delete/list/passwd commands (masked
  interactive password prompt via golang.org/x/term) plus create/delete/
  list subcommands for calendar/addressbook.
- cmd/server/main.go: pre-creates on-disk collections from the DB at
  startup instead of cfg.Users; warns when no users exist yet.
- Updated tests to seed data via the DB; added resources_test.go for the
  new web UI handlers.
- README.md and .github/copilot-instructions.md updated to document the
  new nidusctl commands and the DB-backed architecture.

Verified end-to-end against a live test server: nidusctl user/calendar/
addressbook create, DAV Basic Auth PROPFIND, web login, dashboard
rendering, and web UI create/delete of resources all confirmed working.

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

internal/db/db.go

@@ -30,7 +30,7 @@ func Open(path string) (*DB, error) {
 		}
 	}
 
-	conn, err := sql.Open("sqlite", path)
+	conn, err := sql.Open("sqlite", path+"?_pragma=foreign_keys(1)")
 	if err != nil {
 		return nil, fmt.Errorf("opening database %q: %w", path, err)
 	}
@@ -53,6 +53,28 @@ func (d *DB) Close() error {
 
 func (d *DB) migrate() error {
 	const schema = `
+CREATE TABLE IF NOT EXISTS users (
+	username TEXT PRIMARY KEY,
+	password_hash TEXT NOT NULL,
+	display_name TEXT NOT NULL DEFAULT '',
+	email TEXT NOT NULL DEFAULT '',
+	created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE TABLE IF NOT EXISTS calendars (
+	owner TEXT NOT NULL REFERENCES users (username) ON DELETE CASCADE,
+	name TEXT NOT NULL,
+	created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+	PRIMARY KEY (owner, name)
+);
+
+CREATE TABLE IF NOT EXISTS addressbooks (
+	owner TEXT NOT NULL REFERENCES users (username) ON DELETE CASCADE,
+	name TEXT NOT NULL,
+	created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+	PRIMARY KEY (owner, name)
+);
+
 CREATE TABLE IF NOT EXISTS calendar_shares (
 	id INTEGER PRIMARY KEY AUTOINCREMENT,
 	owner TEXT NOT NULL,