feat(UI-Overhaul-Phase6): Tags Umstrukturierung

Migration 0138: Add parent_id, applicable_to, icon columns to tags table

Backend:
- Tag model: add parent_id (self-FK), applicable_to (JSONB), icon (VARCHAR)
- TagCreate/TagUpdate/TagResponse schemas: add new fields
- Tags routes: create_tag, update_tag, list_tags return new fields

Frontend:
- api/tags.ts: Tag interface, CreateTagPayload, UpdateTagPayload updated with new fields
- Tags route moved from /tags to /settings/tags (under Settings)
- Tags.tsx: TagFormModal updated with parent tag selector, icon picker, applicable_to multi-select
- TagsPage passes tags list to TagFormModal for parent selection

tsc clean, backend import OK

frontend/src/api/tags.ts

@@ -20,6 +20,9 @@ export interface Tag {
   created_at?: string | null;
   updated_at?: string | null;
   usage_count?: number;
+  parent_id?: string | null;
+  applicable_to?: string[] | null;
+  icon?: string | null;
 }
 
 export interface TagAssignment {
@@ -43,12 +46,18 @@ export interface CreateTagPayload {
   name: string;
   color?: string;
   description?: string | null;
+  parent_id?: string | null;
+  applicable_to?: string[] | null;
+  icon?: string | null;
 }
 
 export interface UpdateTagPayload {
   name?: string;
   color?: string;
   description?: string | null;
+  parent_id?: string | null;
+  applicable_to?: string[] | null;
+  icon?: string | null;
 }
 
 export interface AssignTagPayload {