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/schemas.py

@@ -40,4 +40,9 @@ class UserSearchRead(BaseModel):
     id: int
     name: str
     username: str
+    email: EmailStr
     avatar_url: str | None = None
+
+
+class ContactByEmailRequest(BaseModel):
+    email: EmailStr