feat(contacts): switch contacts UX to email-first flow

- expose email in user search/contact responses

- add add-contact-by-email API endpoint

- show email in contacts/search and add contact form by email in Contacts tab

app/users/repository.py

@@ -43,7 +43,12 @@ async def search_users_by_username(
     exclude_user_id: int | None = None,
 ) -> list[User]:
     normalized = query.lower().strip().lstrip("@")
-    stmt = select(User).where(func.lower(User.username).like(f"%{normalized}%"))
+    stmt = select(User).where(
+        or_(
+            func.lower(User.username).like(f"%{normalized}%"),
+            func.lower(User.email).like(f"%{normalized}%"),
+        )
+    )
     if exclude_user_id is not None:
         stmt = stmt.where(User.id != exclude_user_id)
     stmt = stmt.order_by(User.username.asc()).limit(limit)