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/detail.go

@@ -21,13 +21,14 @@ const (
 )
 
 type detailModel struct {
-	entity *db.Entity
-	scroll int
-	height int
-	width  int
-	mode   detailMode
-	run    runModel
-	fill   fillModel
+	entity    *db.Entity
+	backlinks []db.Backlink
+	scroll    int
+	height    int
+	width     int
+	mode      detailMode
+	run       runModel
+	fill      fillModel
 }
 
 func newDetailModel() detailModel {
@@ -36,6 +37,7 @@ func newDetailModel() detailModel {
 
 func (d *detailModel) setEntity(e *db.Entity) {
 	d.entity = e
+	d.backlinks = nil
 	d.scroll = 0
 	d.mode = detailPreview
 }
@@ -144,6 +146,25 @@ func (d detailModel) previewView(width int) string {
 		b.WriteString("\n")
 	}
 
+	if len(d.backlinks) > 0 {
+		b.WriteString("\n")
+		b.WriteString(detailLabelStyle.Render("  ← backlinks"))
+		b.WriteString("\n")
+		for _, bl := range d.backlinks {
+			label := bl.Body
+			if bl.Title != nil {
+				label = *bl.Title
+			} else if len(label) > 40 {
+				label = label[:40] + "…"
+			}
+			line := "    " + backlinkStyle.Render(label)
+			if bl.LinkText != "" {
+				line += " " + hintDescStyle.Render("(as \""+bl.LinkText+"\")")
+			}
+			b.WriteString(line + "\n")
+		}
+	}
+
 	b.WriteString("\n")
 	meta := fmt.Sprintf("created  %s", e.CreatedAt.Format(time.DateTime))
 	if e.ModifiedAt != e.CreatedAt {