fix(backlinks): keep subdomain overview without history

[shashank-sn]

Fixes #291.

Backlink history does not support the include_subdomains scope used by subdomain-inclusive summaries. Requesting both together caused a history failure to discard a valid summary and replace the whole Overview page with an error.

The overview service now requests history only for root-domain lookups. Subdomain and exact-URL summaries return with empty trend arrays, which uses the existing unavailable-history UI and avoids an unsupported billed request.

Regression coverage verifies that:

• subdomain summaries remain available and cache normally without a history request;
• summary failures remain fatal;
• root-domain history failures still propagate and are not cached.

Validation:

• 139 test files and 1,168 tests passed
• Prettier, Knip, TypeScript, badseo TypeScript, Oxlint, and plugin synchronization passed
• worker build passed
• website type check and prerender build passed

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