Fix N+1 query in workflow visualization controller

[Copilot]

Category.find was called inside loops while building lanes and nodes, creating N+1 queries when the category data was already available from the steps collection.

Changes

• Build category lookup hash upfront: Fetch all categories once with Category.where(id: category_ids).index_by(&:id) instead of individual Category.find calls
• Replace inline lookups: Use categories_by_id[step.category_id] in both lanes and nodes construction
• Add nil handling: Filter nil category_id values before query and add guards in loops with .compact

# Before: N queries
lanes = steps.map { |step| category = Category.find(step.category_id); ... }
nodes = steps.map { |step| category_name = Category.find(step.category_id).name; ... }

# After: 1 query
category_ids = steps.map(&:category_id).compact.uniq
categories_by_id = Category.where(id: category_ids).index_by(&:id)
lanes = steps.map { |step| category = categories_by_id[step.category_id]; next unless category; ... }
nodes = steps.map { |step| category = categories_by_id[step.category_id]; next unless category; ... }

Reduces queries from O(n) to O(1) where n = number of workflow steps.

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