Refine API store capability boundaries

[ArionMiles]

Description

Introduce a small consumer-side API store capability slice to show how handler code can depend on narrow interfaces instead of the full Storer surface.

This is intentionally limited to config/status endpoints so the pattern is easy to review before applying it more broadly.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Build/CI configuration change

Related Issue

Fixes #(issue)

Changes Made

• Added narrow API-side store capabilities for app config reads, app config writes, and stats reads.
• Updated status, base-currency, and scan-interval handlers to use those capability interfaces.
• Added compile-time assertions that both store.Store and store.InstrumentedStore satisfy the new capability interfaces.
• Removed stale dupl suppressions after the handler refactor reduced duplication below the lint threshold.

Testing

• Unit tests pass (task test:be)
• Linter passes (task lint:be:prod)
• Manual testing completed
• Added new tests (if applicable)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

Not applicable.

Additional Notes

The broader Storer interface remains in place. This PR is a small representative slice for validating the boundary shape before extending it to other API groups.