Add reusable dataset metadata fields

[karelxfi]

What changed

• add official website, docs, and reviewed explorer URLs to dataset metadata
• add explicit tier and private fields
• add the logo_bg: white rendering hint for dark logos
• document the metadata contract
• extend the metadata CLI and validate every metadata record

Every chain-level value is repeated on its networks and validated for consistency within the same ecosystem.

Why

Consumers currently have to copy the sqd.dev enrichment layer to get chain resources, support tier, access scope, and logo treatment. These fields belong with the dataset identity metadata so other projects can consume one maintained source.

Field naming

The source metadata uses snake_case, so the website's logoBg value is represented as logo_bg alongside existing fields such as display_name and logo_url.

Validation

• python scripts/sqd-network-metadata/__main__.py validate (215 metadata records and 196 declared datasets)
• python -m py_compile scripts/sqd-network-metadata/*.py
• exercised the add-command metadata builder with all new fields
• checked 281 unique website/docs/explorer URLs; omitted three optional explorer links that returned HTTP 404
• git diff --check

Dependency

Stacked on #164 so this PR contains only the new fields and tooling changes. Retarget to main after #164 merges.