feat(#221): add V39 migration for tag hierarchy and colors

Adds parent_id FK (ON DELETE SET NULL), self-reference check constraint,
parent_id index, and nullable color column to the tag table.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/db/migration/V39__add_tag_hierarchy.sql

@@ -0,0 +1,9 @@
+-- Add self-referencing parent FK for tag hierarchy (adjacency list model).
+-- ON DELETE SET NULL: deleting a parent promotes its children to root level.
+ALTER TABLE tag ADD COLUMN parent_id UUID REFERENCES tag(id) ON DELETE SET NULL;
+ALTER TABLE tag ADD CONSTRAINT chk_tag_no_self_reference CHECK (parent_id != id);
+CREATE INDEX idx_tag_parent_id ON tag(parent_id);
+
+-- Optional color token (e.g. "sage", "teal") for root-level tags.
+-- Validated against the allowed palette in TagService before save.
+ALTER TABLE tag ADD COLUMN color VARCHAR(20);