Subtask: reduce Fallow duplication baseline through targeted refactoring

[eviltester]

Summary

Refactor the codebase to reduce the accepted Fallow duplication baseline over time.

This follows the Fallow adoption work in #262. Fallow dupes is now configured and running in CI using a regression baseline, but the current accepted duplication level is still high enough that we should gradually reduce it with targeted refactoring.

Scope

• review the current Fallow duplication findings and identify the highest-value clone groups
• prioritize duplicate logic in application code, API request handling, shared controller behavior, and repeated schema/config shaping before low-value boilerplate
• refactor duplicate code where extraction improves maintainability without making the code harder to understand
• suppress or explicitly defer clone groups that are intentionally repetitive and not worth abstracting
• regenerate and commit a lower dupes regression baseline after each meaningful reduction pass
• document any conventions for when duplication should be accepted versus refactored

Acceptance Notes

• the committed Fallow duplication baseline is lower than it is today
• the reduction comes from real code improvements, not broad suppressions that hide useful signal
• any intentional suppressions are narrowly scoped and explained
• pnpm run fallow:ci still passes with the updated baseline