ci: add Dependabot config (uv + GitHub Actions)

[coelhogonzalo]

What

Adds .github/dependabot.yml with two update streams, both targeting develop:

• uv — Python dependencies via pyproject.toml + uv.lock (Dependabot supports uv natively; the pip ecosystem would not manage our lockfile).
• github-actions — the actions used in .github/workflows (checkout, setup-uv, setup-python).

Config choices

• Monthly, grouped updates — every merge into develop triggers a beta publish, so all version bumps arrive as a single grouped PR per run instead of one PR per dependency.
• 7-day cooldown — for wallet software handling keys (lwk, bdkpython, wallycore, coincurve), waiting out just-published releases reduces supply-chain risk; compromised releases are usually caught within days.
• Majors ignored, minors allowed — majors need a human decision. Minors are kept (deviating from the reference config this was based on, which was patch-only) because most of our deps are 0.x where the meaningful releases are semver-minor (e.g. lwk 0.18 → 0.19).
• target-branch: develop — follows the develop-first flow. Note GitHub always sends security update PRs to the default branch regardless of this setting.
• Commit prefixes deps:/ci: with scope, labeled dependencies + ecosystem.

CI note: the Tests workflow needs no secrets, so Dependabot PRs get full CI despite the restricted GITHUB_TOKEN.

[coelhogonzalo]

I thought it might be a good idea. Your call to merge