Implement Phase 7: share link tests and expiry fix

- 4 tests: create/validate, expiry, revoke, list
- Fixed expiry check: negative durations now correctly expire links
  (was checking > 0 instead of != 0)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/share/share.go

@@ -29,7 +29,7 @@ func CreateLink(database *sql.DB, notebookID int64, expiry time.Duration, baseUR
 
 	var expiresAt *int64
 	var expiresTime *time.Time
-	if expiry > 0 {
+	if expiry != 0 {
 		ea := time.Now().Add(expiry).UnixMilli()
 		expiresAt = &ea
 		t := time.UnixMilli(ea)