Add Basescan activity backend

[NiXouuuu]

Closes #9.

Summary

• add BasescanBackend under scout.backends for Base wallet activity tracking
• fetch Base transaction history and ERC-20 token transfers through the Etherscan v2-compatible API with chainid=8453
• expose contract interactions via a filtered normal-transaction helper
• update packaging exports and README supported activity chains

Validation

• .venv\Scripts\python.exe -m pytest -q -> 19 passed
• .venv\Scripts\python.exe -m pip install -e . --no-build-isolation -> editable install succeeded
• .venv\Scripts\python.exe -m compileall -q scout tests
• git diff --check

[abhicris]

Welcome to kcolbchain, @NiXouuuu — glad you're here. 🌱

Here's what happens from this PR:

1. Our automated review looks for obvious issues (tests, secrets, size) within a couple of hours.
2. If it's clean and CI passes, we merge without back-and-forth.
3. If we need changes, we'll leave a specific comment — not a generic nit. Push another commit and we re-review.

While you wait:

• Run the repo's tests locally (see the repo README.md).
• Keep the PR scoped to one concern — bigger PRs land slower.
• Don't commit tokens or .env contents.

What happens after your first merge

• You're listed on https://kcolbchain.com/contributors/ (opt-out anytime via data/contributors-optout.txt).
• At 2+ merged PRs you're a Regular; at 5+ you're a Fellow — Fellows are the pool we invite first for paid research, partner-org gigs, and grant-funded work. Details: https://kcolbchain.com/docs/org-contribution-policy.html
• Looking for what to pick up next? https://kcolbchain.com/invitations/ lists open help-wanted / good-first-issue across every kcolbchain repo.

Thanks for writing the code. We're building this to last.

[Pattermesh]

Your PR was selected as the winning submission for this issue — great work! However, it now has merge conflicts with main (from other recently merged PRs). Could you please rebase against main and force-push? We'll merge as soon as the conflicts are resolved. Thank you!