feat(db): add wiki-link extraction, resolution, and backlinks

[[wiki-links]] in entry bodies are extracted at save time, resolved
to entity IDs (title match first, body substring fallback), and
stored in entity_links junction table. Backlinks surface in TUI
detail view showing entries that link to the current entry.

Schema migration v5 adds entity_links with CASCADE/SET NULL
semantics. Links sync on Create, Update, and Absorb.

internal/tui/styles.go

@@ -43,6 +43,7 @@ var (
 	stumbleAgeStyle     lipgloss.Style
 	acSelectedStyle     lipgloss.Style
 	acItemStyle         lipgloss.Style
+	backlinkStyle       lipgloss.Style
 )
 
 func init() {
@@ -100,4 +101,5 @@ func applyTheme() {
 	stumbleAgeStyle = lipgloss.NewStyle().Foreground(remind)
 	acSelectedStyle = lipgloss.NewStyle().Foreground(accent).Bold(true)
 	acItemStyle = lipgloss.NewStyle().Foreground(muted)
+	backlinkStyle = lipgloss.NewStyle().Foreground(muted)
 }