Add python-dotenv to the http dev dependencies and make live tests skip without credentials #25

packages/http/tests/conftest.py imports python-dotenv, but no dev dependency list declares it, so a fresh pip install -e ./packages/http[dev] cannot run the tests.

The live tests in packages/http/tests/test_activities.py and test_queries.py fail rather than skip when TURNKEY_API_PUBLIC_KEY and friends are not set.

Follow-up: add python-dotenv to the http dev extra, and add a module-level skip when the credentials are missing so offline runs stay green. The offline tests added in #21 already run without credentials.