chore: add dependabot config (pip + github-actions)

[karlwaldman]

Summary

Adds .github/dependabot.yml (weekly pip + github-actions updates, Monday, 5-PR cap, labelled dependencies). The repo previously had no Dependabot coverage.

Context from the dependency review that prompted this:

• Runtime deps in pyproject.toml are floor-pinned ranges (httpx>=0.24, pydantic>=2, python-dateutil>=2.8) — PyPI users already resolve to current releases, nothing stale to bump.
• pip-audit on a fresh dev install: no known vulnerabilities.
• Weekly-health already runs pip-audit --strict (continue-on-error); Dependabot closes the gap for Actions pins and the dev toolchain.

No code changes; config only.

🤖 Generated with Claude Code

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: ab492973-1c42-488c-97aa-c4a6e58fe728

📥 Commits

Reviewing files that changed from the base of the PR and between 6cfa2d8 and 9f307df.

📒 Files selected for processing (1)

• .github/dependabot.yml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/add-dependabot

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