Pull the StartMigration -> Migrate -> FinishMigration orchestration out of
the v1 echo handlers into handler.RunFileMigration and csv.RunMigration so
the v2 API can reuse the exact same business logic. v1 is refactored onto
them and stays byte-identical on the wire.
Also tag the CSV detect/preview/config DTOs with doc:/enum: so they carry
descriptions in the v2 OpenAPI schema (ignored by v1 swaggo/xorm).
This changes the error handling to a centralized HTTP error handler in `pkg/routes/error_handler.go` that converts all error types to proper HTTP responses. This simplifies the overall error handling because http handler now only need to return the error instead of calling HandleHTTPError as previously.
It also removes the duplication between handling errors with and without Sentry.
🐰 Hop along, dear errors, no more wrapping today!
We've centralized handlers in a shiny new way,
From scattered to unified, the code flows so clean,
ValidationHTTPError marshals JSON supreme!
Direct propagation hops forward with glee,
A refactor so grand—what a sight to see! 🎉