fix: empty _Row truthy bug in add_contact UPSERT

fetchone() returns an empty _Row on no match (no __bool__), so  was
always true and UPDATE ran with a nonexistent id, silently inserting nothing.
Changed to  so missing rows fall through to INSERT.

src/vm_web.py

@@ -511,7 +511,7 @@ async def add_contact(request: Request, vm_session: str = Cookie(default=None)):
         row = con.execute(
             "SELECT id FROM contacts WHERE number_e164 = ?", (normalized,)
         ).fetchone()
-        if row:
+        if row and row.get("id"):
             con.execute(
                 "UPDATE contacts SET name=?, email=?, updated_at=NOW() WHERE id=?",
                 (name, email, row["id"]),