Add nidusctl admin CLI for managing calendar/address-book shares

- tools/nidusctl: subcommands `calendar {share,unshare,shares}` and
  `addressbook {share,unshare,shares}`, thin wrapper around internal/db.
  Warns (non-fatal) if owner/user isn't in config.yaml.
- internal/db: add SharesOfAddressBook (owner-perspective query, mirrors
  SharesOfCalendar) needed by the CLI.
- internal/db: db.Open now creates the parent data directory itself, so
  the CLI works standalone without requiring the server to have run first.
- Update Makefile (build bin/nidusctl, new 'nidusctl' target), README and
  copilot-instructions with usage docs.

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

internal/db/db.go

@@ -8,6 +8,8 @@ package db
 import (
 	"database/sql"
 	"fmt"
+	"os"
+	"path/filepath"
 
 	_ "modernc.org/sqlite"
 )
@@ -19,8 +21,15 @@ type DB struct {
 }
 
 // Open opens (creating if necessary) the SQLite database at path and runs
-// schema migrations.
+// schema migrations. The parent directory of path is created if it
+// doesn't already exist.
 func Open(path string) (*DB, error) {
+	if dir := filepath.Dir(path); dir != "" && dir != "." {
+		if err := os.MkdirAll(dir, 0o755); err != nil {
+			return nil, fmt.Errorf("creating database directory %q: %w", dir, err)
+		}
+	}
+
 	conn, err := sql.Open("sqlite", path)
 	if err != nil {
 		return nil, fmt.Errorf("opening database %q: %w", path, err)

internal/db/shares.go

@@ -178,6 +178,33 @@ func (d *DB) UnshareAddressBook(owner, bookName, sharedWith string) error {
 	return nil
 }
 
+// SharesOfAddressBook lists everyone owner's address book bookName has
+// been shared with.
+func (d *DB) SharesOfAddressBook(owner, bookName string) ([]AddressBookShare, error) {
+	rows, err := d.conn.Query(`
+		SELECT owner, addressbook_name, shared_with, permission
+		FROM addressbook_shares
+		WHERE owner = ? AND addressbook_name = ?
+		ORDER BY shared_with`,
+		owner, bookName)
+	if err != nil {
+		return nil, fmt.Errorf("listing address book shares: %w", err)
+	}
+	defer rows.Close()
+
+	var shares []AddressBookShare
+	for rows.Next() {
+		var s AddressBookShare
+		var perm string
+		if err := rows.Scan(&s.Owner, &s.AddressBookName, &s.SharedWith, &perm); err != nil {
+			return nil, fmt.Errorf("scanning address book share: %w", err)
+		}
+		s.Permission = Permission(perm)
+		shares = append(shares, s)
+	}
+	return shares, rows.Err()
+}
+
 // AddressBooksSharedWith lists all address books (from any owner) that
 // have been shared with user.
 func (d *DB) AddressBooksSharedWith(user string) ([]AddressBookShare, error) {