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/db/db.go

@@ -56,7 +56,7 @@ func (s *Store) Backup(dst string) error {
 	return err
 }
 
-const currentSchema = 4
+const currentSchema = 5
 
 var migrations = []func(db *sql.DB) error{
 	// v1: initial schema
@@ -188,6 +188,20 @@ var migrations = []func(db *sql.DB) error{
 		}
 		return nil
 	},
+
+	// v5: add entity_links table for wiki-links
+	func(db *sql.DB) error {
+		_, err := db.Exec(`
+			CREATE TABLE entity_links (
+				from_id   TEXT NOT NULL REFERENCES entities(id) ON DELETE CASCADE,
+				to_id     TEXT REFERENCES entities(id) ON DELETE SET NULL,
+				link_text TEXT NOT NULL,
+				PRIMARY KEY (from_id, link_text)
+			);
+			CREATE INDEX idx_entity_links_to ON entity_links(to_id) WHERE to_id IS NOT NULL;
+		`)
+		return err
+	},
 }
 
 func (s *Store) migrate() error {