fix: correct email-in-address format, remove dead code, update tests (#110)

- Fix email format in AuthService.get_email_in_address to use
  receipts+{token}@receipts.cartsnitch.com (was broken: @email.cartsnitch.com)
- Remove dead EmailInAddressResponse class and GET /auth/me/email-in-address
  endpoint from auth/routes.py (endpoint moved to routes/user.py)
- Add instructions field to EmailInAddressResponse schema
- Update routes/user.py to include instructions in the response
- Update test URLs from /auth/me/email-in-address to /api/v1/me/email-in-address

Co-authored-by: CartSnitch Engineer Bot <cartnoreply@cartsnitch.com>
Co-authored-by: Paperclip <noreply@paperclip.ing>

src/cartsnitch_api/services/auth.py

@@ -76,4 +76,4 @@ class AuthService:
         if not user:
             raise LookupError("User not found")
 
-        return f"{user.email_inbound_token}@email.cartsnitch.com"
+        return f"receipts+{user.email_inbound_token}@receipts.cartsnitch.com"