feat: add isAiPromptDeleted function to filter out deleted AI prompts

[rachel-kittens]

Problem

When users deleted AI prompts through the UI, those prompts continued to be sent to Mystique in subsequent weekly runs.

Investigation

Data flow

• AI prompts originate from Ahrefs keyword data, stored in weekly parquet files by spacecat-import-worker
• spacecat-audit-worker reads parquet files and sends prompts to Mystique for categorization
• Mystique categorizes prompts and stores them in config.aiTopics
• UI displays prompts from config.aiTopics and allows users to delete them
• Deleted prompts are recorded in config.deleted.prompts

Root cause

• Parquet files are immutable
• When user deletes an AI prompt, it's added to config.deleted.prompts with the Mystique-assigned category
• Parquet file still contains the prompt with category: "" (empty string)
• The audit worker was not filtering parquet prompts against deleted.prompts
• Even if it did filter, category mismatch would prevent matching:
  • Parquet: {prompt: "...", category: ""}
  • Deleted: {prompt: "...", category: "Breakfast Cereals"}
• Not an issue for human prompts: UI removes deleted regions from config.topics before saving → no filtering needed in audit worker

Solution

Added filtering that:

1. Only matches deleted prompts with origin: 'ai'
2. Matches on prompt text + region only (ignores category/topic)
3. Filters AI prompts loaded from parquet before sending to Mystique

Note: aiTopics remains unchanged

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Related Issues

LLMO-2627

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This PR will trigger a minor release when merged.