feat(products): configure products and GitHub App integration

Summary

• Add configurable products with GitHub repository associations, including the Ecto schema, Products context, migration, and seeded sample products.
• Add a settings products page with a full-page table, Noora modal, and searchable GitHub App repository dropdown.
• Add GitHub App installation repository search plus signed webhook verification at /webhooks/github.
• Wire Tuist production External Secrets refs for the GitHub App credentials and webhook secret.

Testing

• mix test
• mix compile --warnings-as-errors
• mix credo
• mix assets.build
• helm template hive infra/helm/hive -f infra/helm/hive/values-production.yaml

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