Clarify frontend API migration with exact backend contracts

[jonasyr]

Summary

• rewrite the frontend migration guide as a drop-in reference aligned with the current backend routes and shared types
• document validated query parameters, response schemas, and before/after fetch patterns for each repository endpoint
• add error-handling guidance and a migration checklist to ensure clients honor pagination, filters, and validation flags

Testing

• Not run (documentation-only changes)

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Codex Task