Add single-breaker CRUD endpoints for better UX

Problem

Managing breakers currently requires clients to:

1. Fetch the entire breaker configuration
2. Edit it manually
3. Send the whole thing back via PUT

This creates a poor experience for users. Even small changes (like toggling a breaker or adjusting a threshold) require full-document updates, which feels heavy, error-prone, and unintuitive.

This has become a clear adoption blocker: most users expect to edit a single breaker directly, not manipulate the entire configuration blob.

Proposed Solution

Add single-breaker CRUD endpoints that wrap the existing internal flow:

• POST /v1/projects/:project_id/breakers - Create a single breaker
• GET /v1/projects/:project_id/breakers/:breaker_id - Get a single breaker
• PATCH /v1/projects/:project_id/breakers/:breaker_id - Update specific fields on a breaker
• DELETE /v1/projects/:project_id/breakers/:breaker_id - Delete a single breaker

Implementation Notes

• Keep the internal "single configuration document" model — it simplifies validation and consistency
• New endpoints wrap the existing bulk replace flow under the hood
• Externally, users get a friendlier API surface that feels straightforward and conventional

Acceptance Criteria

• [ ] Single-breaker endpoints implemented
• [ ] PATCH supports partial updates (only send fields you want to change)
• [ ] Proper error handling (404 for missing breaker, 409 for conflicts, etc.)
• [ ] Documentation updated
• [ ] Tests added