feat: OCI manifest size badge (v1.1.0)

[Will-Luck]

Summary

• Fetch per-platform compressed image sizes from the GHCR OCI registry API (ghcr.io/v2/)
• Size badge now shows per-platform breakdown: 82.5 MB (amd64) | 81.2 MB (arm64)
• Version-aware manifest fetching: tries scraped version tag first, falls back to latest
• Best-effort resilience: transient failure on one platform doesn't lose data for others

Closes #1

Test plan

• 17/17 unit tests passing (6 new: 2 manifest parsing, 4 badge formatting)
• Live smoke test: docker-sentinel 11.6 MB (amd64) | 11.4 MB (arm64)
• Live smoke test: iplayer-arr 82.5 MB (amd64) | 81.2 MB (arm64)
• Badge message length verified for shields.io (35 chars, renders cleanly)