README: CI badge; pyproject: align keywords with marketing brief

[cigan1]

Summary

Two small discoverability fixes flagged during the siftfy.io SEO audit (Task 12 of the marketing brief).

What changed

• README.md — CI build-status badge added between Python-versions and License. Of the brief's three required badges (PyPI version, build status, license), this was the only one not already shown.
• pyproject.toml — keywords extended with spam-api, spam-classification, and moderation. Existing entries (spam-classifier, api-client, siftfy, content-moderation, spam, spam-detection) stay — they don't hurt and a few are higher-traffic PyPI search terms than the brief's exact wording.

What was already done (and didn't need changes)

• ✅ PyPI publication — pypi.org/pypi/siftfy/json returns 200; the package is live.
• ✅ Trusted-publishing-on-tag workflow at .github/workflows/publish.yml (verifies tag matches _version.py before pushing).
• ✅ README sections: install, quickstart, async, calibrated probabilities, errors (with retry semantics), configuration (with httpx.Client injection), resources, license.
• ✅ MIT LICENSE file at root.
• ✅ [project.urls] table: Homepage, Documentation, Source, Issues, Changelog.
• ✅ CI matrix workflow runs ruff + mypy + pytest across 3.9–3.13.

Test plan

• After merge, the CI badge in README renders green when the CI workflow is passing on main.
• PyPI search for "spam-api" surfaces this package once the next version is published (keywords are baked into the wheel/sdist at build time, so they take effect on the next tagged release).

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