fix: stop configuring root logging at import (#100)

[tschm]

Fixes #100

Acceptance criterion (verbatim):

python -c "import logging, pycharting; print(logging.getLogger().handlers)" prints [], a test asserts that, and make test passes.

What changed

• src/pycharting/core/server.py: removed the module-level logging.basicConfig(level=logging.INFO, ...). A plain import pycharting no longer adds a handler to the root logger or changes its level, so it can't override the logging setup of whatever app imports it.
• tests/pycharting/core/test_server.py: added test_import_leaves_root_logging_unconfigured. It runs the criterion's exact command in a fresh interpreter, because pytest attaches its own capture handlers to the root logger and an in-process check would never see [].

Gates

• make fmt: pass
• make test: pass, 179 tests, 100% coverage (bar is 90%)
• The criterion's command, run by hand: prints []

Deliberately not addressed. The issue's optional suggestions are not in this PR: attaching a NullHandler to the pycharting logger ("if default output is wanted"), and adding logging config to demo.py or to server.py's __main__ block. The criterion doesn't need either. uvicorn.run(..., log_level="info") still configures uvicorn's own server logs when you run the server directly.

Closes #100

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