Problem
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The codebase currently stores the Candlepin consumer UUID in TWO places:

• content_facet.uuid
• subscription_facet.uuid

This duplication causes several issues:

1. Data inconsistency: content_facet.uuid can be nil while subscription_facet.uuid is present (see commit b187e48)
2. Code confusion: Developers must choose which UUID to use
3. Bugs: Container gateway sync failed due to nil content_facet.uuid (issue #38862)
4. Architectural violation: UUID is a subscription/consumer concept (from Candlepin), not a content concept

Historical Context
------------------
This duplication originated in the 2015 "Host Unification" migration (PR #5528, issue #11500) when the old System model was split into content_facet and subscription_facet. Both facets received the same UUID during migration, but this was never cleaned up.

Current Usage
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subscription_facet.uuid (17+ locations):

• All Candlepin API calls
• Subscription management
• Remote execution
• Container registry

content_facet.uuid (4 locations - should be migrated):
1. app/models/katello/host_collection.rb:54 - Getting consumer IDs
2. app/services/katello/host/package_profile_uploader.rb:28 - Validation check
3. app/services/katello/host/profiles_uploader.rb:14 - Validation check
4. app/controllers/katello/api/registry/registry_proxies_controller.rb:55, 82 - Client cert auth

Proposed Solution
-----------------
1. Migrate all 4 remaining content_facet.uuid usages to subscription_facet.uuid
2. Add database migration to copy any remaining content_facet.uuid values to ensure no data loss
3. Remove uuid column from katello_content_facets table
4. Add delegation method if needed for compatibility: content_facet.uuid → subscription_facet.uuid

Related Issues/Commits
----------------------

• Commit b187e48: "Fixes #38862 - Switch to subscription facet for uuids"
• Original migration: commit ccc3dc7c (2015) - PR #5528, issue #11500