Add setting for custom registries for Foundational Flows

What does this MR do and why?

This MR adds database and backend support for a new instance setting for self-managed instances to set a custom registry used for Duo Agent Platform images.

Migrations

bin/rails db:migrate:up:main VERSION=20260205084104
main: == [advisory_lock_connection] object_id: 133840, pg_backend_pid: 65065
main: == 20260205084104 AddDuoWorkflowsDefaultImageRegistryToApplicationSettings: migrating 
main: -- add_column(:application_settings, :duo_workflows_default_image_registry, :text)
main:    -> 0.0037s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- execute("ALTER TABLE application_settings\nADD CONSTRAINT check_6e7bf3aef3\nCHECK ( char_length(duo_workflows_default_image_registry) <= 512 )\nNOT VALID;\n")
main:    -> 0.0030s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0004s
main: -- execute("ALTER TABLE application_settings VALIDATE CONSTRAINT check_6e7bf3aef3;")
main:    -> 0.0022s
main: -- execute("RESET statement_timeout")
main:    -> 0.0005s
main: == 20260205084104 AddDuoWorkflowsDefaultImageRegistryToApplicationSettings: migrated (0.1117s) 

main: == [advisory_lock_connection] object_id: 133840, pg_backend_pid: 65065

bin/rails db:migrate:down:main VERSION=20260205084104
main: == [advisory_lock_connection] object_id: 133820, pg_backend_pid: 64505
main: == 20260205084104 AddDuoWorkflowsDefaultImageRegistryToApplicationSettings: reverting 
main: -- remove_column(:application_settings, :duo_workflows_default_image_registry)
main:    -> 0.0097s
main: == 20260205084104 AddDuoWorkflowsDefaultImageRegistryToApplicationSettings: reverted (0.0411s) 

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

It's probably easiest to follow the instructions from the frontend MR: !222283 (merged)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #584689 (closed)