feat(llm-content-coverage): add LLM content coverage audit

[anagarwa]

Summary

• Adds new llm-content-coverage audit type that identifies topics where AI prompt volume is high but brand citation is low
• Fetches up to 200 brand topics from rpc_brand_presence_topics (Mysticat PostgREST), scores each with a gap score (promptCount² / (promptCount + brandCitations + brandMentions)), and selects the top 10 candidates
• Calls Semrush FanoutService via gRPC to find low-rank keywords (brand ranks at position > 5) for each topic
• Post-processor creates/updates an Opportunity with LLMO topic data + Semrush fanout results, and syncs one Suggestion per low-rank keyword
• Registers handler in src/index.js as llm-content-coverage

Depends on

Must merge and publish spacecat-shared#1776 first — this PR imports getGrpcClients and fetchLowRankFanoutKeywords from @adobe/mysticat-shared-seo-client.

Test plan

• Audit validated end-to-end on adobe.com (siteId c2473d89-e997-458d-a86d-b4096649c12b)
  • 194 LLMO topics fetched, top 10 scored by gap score
  • FanoutService returned 10/10 topic matches
  • Opportunity created with topics + semrushTopics in data field
  • 1 keyword suggestion synced: "how to use generative ai" (volume=480, brandPosition=6)
• SEO_CLIENT_ID / SEO_CLIENT_SECRET loaded from Vault at path bootstrapEnvironment/development/spacecat-services--audit-worker/anagarwa
• Unit tests to be added (coverage enforcement will catch on CI)

Opportunity data shape

{
  "data": {
    "domain": "adobe.com",
    "topics": [{ "topic": "...", "promptCount": 41, "brandCitations": 0, "brandMentions": 66, "gapScore": 16 }],
    "semrushTopics": [{ "topic": "...", "matchedTopicName": "Generative AI", "similarityScore": 93, "keywords": [{ "keyword": "how to use generative ai", "volume": 480, "brandPosition": 6 }] }]
  }
}

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