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

alembic/versions/0138_tags_tree_structure.py

@@ -0,0 +1,42 @@
+"""Tags: parent_id, applicable_to, icon columns
+
+Revision ID: 0138
+Revises: 0137
+Create Date: 2026-08-21
+
+Adds parent_id for tree structure, applicable_to for entity-type filtering,
+and icon for per-tag icon selection.
+"""
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import UUID as PGUUID, JSONB
+
+# revision identifiers, used by Alembic.
+revision = "0138"
+down_revision = "0137"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # parent_id for tree structure (self-referencing FK)
+    op.add_column("tags", sa.Column("parent_id", PGUUID(as_uuid=True), nullable=True))
+    op.create_foreign_key(
+        "fk_tags_parent_id", "tags", "tags", ["parent_id"], ["id"], ondelete="SET NULL"
+    )
+    op.create_index("ix_tags_parent", "tags", ["parent_id"])
+
+    # applicable_to: list of entity types where this tag can be applied
+    op.add_column("tags", sa.Column("applicable_to", JSONB, nullable=True))
+
+    # icon: icon name for frontend display
+    op.add_column("tags", sa.Column("icon", sa.String(50), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("tags", "icon")
+    op.drop_column("tags", "applicable_to")
+    op.drop_index("ix_tags_parent", table_name="tags")
+    op.drop_constraint("fk_tags_parent_id", "tags", type_="foreignkey")
+    op.drop_column("tags", "parent_id")