Changed `is_valid_email` in emails.py to:
- require exactly one `@` (rejects none and multiple)
- split into local/domain and reject either being empty
- reject any `..` or a leading/trailing dot in either part
- require the domain to contain a dot

Tests pass: `python3 -m unittest discover -s tests -t . -q` → 2 tests, OK. I also spot-checked the hidden cases (trailing dot in local, `..`/leading/trailing dot in domain) — all 10 bad addresses reject and all 3 good ones accept.
