Eliminate N+1 queries in workflow visualization controller

[Copilot]

Addresses N+1 query issue from PR #9 review feedback where Category.find and WorkflowStep.find were called inside loops.

Changes

• Pre-load categories: Single query loads all required categories, indexed by ID for O(1) lookup
• Index workflow steps: Build hash of steps by ID to eliminate repeated WorkflowStep.find calls
• Optimize lane lookups: Replace O(n×m) find_index calls with O(1) hash lookup
• Add nil safety: Guard against missing category/step references with early returns

Before/After

# Before: N+1 queries in loops
lanes = steps.map do |step|
  category = Category.find(step.category_id)  # Query per step
  { name: category.name, link: "/c/#{step.category_id}" }
end

# After: Single query upfront
category_ids = steps.filter_map(&:category_id).uniq
categories_by_id = Category.where(id: category_ids).index_by(&:id)

lanes = steps.map do |step|
  category = categories_by_id[step.category_id]  # Hash lookup
  next unless category
  { name: category.name, link: "/c/#{step.category_id}" }
end.compact

Query count reduced from 1 + N + M to 3 regardless of workflow size (where N = steps, M = step options).

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