refactor: simplify db and cmd flags, reduce code duplication

- Abstract property flags into addPropertyFlags in cmd/root.go and apply to commands
- Remove unused GetUserByUsername from db.go (it's redundant with service.resolveUserIDByName)
- Use configured user in WithMentions directly in inbox command
- Refactor resolve methods in node_service_sqlite.go to reduce duplication

db/db.go

@@ -70,14 +70,3 @@ func Open(path string) (*DB, error) {
 	return &DB{DB: db, path: path}, nil
 }
 
-func (db *DB) GetUserByUsername(username string) (string, error) {
-	var id string
-	err := db.QueryRow(`
-		SELECT n.id FROM nodes n
-		JOIN tags t ON n.id = t.node_id
-		WHERE n.title = ? AND t.tag = '_type::user'`, username).Scan(&id)
-	if err == sql.ErrNoRows {
-		return "", nil
-	}
-	return id, err
-}