fix(backend): store an empty tag colour as NULL

The PATCH "clear colour" path sent an empty string, which violates the
hex CHECK constraint and never falls back to the category colour. Map ''
to NULL via NULLIF in the tag insert/update so a cleared or omitted
colour is stored as NULL.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/db/postgres/tag_repo.go

@@ -272,7 +272,7 @@ func (r *TagRepo) Create(ctx context.Context, t *domain.Tag) (*domain.Tag, error
 	const query = `
 WITH ins AS (
     INSERT INTO data.tags (name, notes, color, category_id, metadata, creator_id, is_public)
-    VALUES ($1, $2, $3, $4, $5, $6, $7)
+    VALUES ($1, $2, NULLIF($3, ''), $4, $5, $6, $7)
     RETURNING *
 )
 SELECT
@@ -321,7 +321,7 @@ WITH upd AS (
     UPDATE data.tags SET
         name        = $2,
         notes       = $3,
-        color       = $4,
+        color       = NULLIF($4, ''),
         category_id = $5,
         metadata    = COALESCE($6, metadata),
         is_public   = $7