fix(web): clarify empty Bing backlink sync state

[arberx]

Summary

Clarifies the Bing backlinks empty state so a synced zero-count window no longer claims every inbound link was filtered as crawler/proxy traffic.
The dashboard now distinguishes true zero raw Bing rows from all-filtered crawler/proxy rows, and shows the hidden crawler/proxy count when applicable.
Added web regression coverage for both empty synced states.

Checks

• pnpm --filter @ainyc/canonry-web test -- backlinks-section.test.tsx
• pnpm --filter @ainyc/canonry-web run typecheck
• pnpm --filter @ainyc/canonry-web run lint
• pnpm --filter @canonry/canonry test -- bing-backlinks-sync.test.ts backlinks-commands.test.ts
• pnpm --filter @ainyc/canonry-api-routes test -- backlinks.test.ts doctor-backlinks.test.ts
• pnpm --filter @ainyc/canonry-integration-bing test -- bing-client.test.ts