feat(cards): add 'note' card type for readable markdown content

New card type renders body as styled markdown with no copy/fill/run
affordance. Glyph: ¶, color: --note.

Migration uses transaction to safely rebuild table constraint.
Checks both 'note' presence and modified_at column to catch
partial migration state.

internal/db/entities.go

@@ -27,6 +27,7 @@ const (
 	CardChecklist CardType = "checklist"
 	CardDecision  CardType = "decision"
 	CardLink      CardType = "link"
+	CardNote      CardType = "note"
 )
 
 func ValidGlyph(s string) bool {
@@ -39,7 +40,7 @@ func ValidGlyph(s string) bool {
 
 func ValidCardType(s string) bool {
 	switch CardType(s) {
-	case CardSnippet, CardTemplate, CardChecklist, CardDecision, CardLink:
+	case CardSnippet, CardTemplate, CardChecklist, CardDecision, CardLink, CardNote:
 		return true
 	}
 	return false