Creates a new tag for the authenticated account, optionally nested under an existing parent tag. The submitted tag_name is normalized server-side before storage: each ~ is replaced with a space, runs of one or more commas are replaced with a single hyphen, Unicode control and format characters (newline, NUL, BOM, etc.) are stripped, and surrounding whitespace is trimmed. This keeps the stored name safe across the system's ~- and ,-delimited tag pipelines (click-tracked URLs, comma-separated tag-list inputs); a caller who submits foo,bar will see foo-bar in the response. A tag_name that is empty after this normalization (only whitespace or control characters) is rejected with 422.