chore: bump version to v0.3.1

[jdwit]

Summary

Patch release covering the three PRs merged since v0.3.0.

Suggested release notes

Highlights

• OAuth scope: yt-analytics-monetary.readonly (feat(auth): request yt-analytics-monetary scope for revenue metrics #21, thanks @maxhuk) - revenue metrics like estimatedRevenue, grossRevenue, cpm, adImpressions now work in analytics query. Without this scope they failed with 401 Insufficient permission. Existing users must re-run ytstudio login to re-consent; if oauthlib complains about scope changes, OAUTHLIB_RELAX_TOKEN_SCOPE=1 ytstudio login resolves it.
• ytstudio videos categories (feat: videos categories command #22) - list assignable YouTube video categories for a region (default US), so you can pick the right category_id for your sidecar without grepping unofficial tables. --region NL, -o json supported.
• Mermaid diagrams in docs (fix(docs): enable mermaid diagrams via pymdownx.superfences custom_fences #20) - the documentation site at https://jdwit.github.io/ytstudio-cli/ now renders mermaid blocks via pymdownx.superfences.

Breaking changes

• Upload sidecar category_id is now required (part of feat: videos categories command #22). The previous default of "22" (People & Blogs) was our own pick, not a YouTube default - hiding it behind a default made it easy to bulk-upload into the wrong category by accident. Sidecars without category_id now fail validation upfront. Add the field; use ytstudio videos categories to look up the right ID.

Test plan

• uv sync + uv run pytest clean (205 tests)
• After merge: tag v0.3.1 on the merge commit and create the GitHub release using the notes above