Export and import data store configurations

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Summary

Adds Export and Import to the Edit Data Store page so the table rules and endpoints built against one data store (typically dev) can be carried over to another (typically prod), on the same server or a different one.

• GET api/datastores/{id}/export returns an indented JSON document with every configuration, its tables and its endpoints. The data store's own connection details are left out.
• POST api/datastores/{id}/import creates each configuration in the document as a new configuration. Existing configurations are never modified: a name already in use is refused, and the import dialog flags the clash and asks for a different name before sending. Validation runs before anything is written, so the import is all-or-nothing.
• Endpoints are imported only when "Include endpoints" is ticked. They are created unpublished and with fresh ids, since the id is the URL clients sync against and the target must hand out its own.
• Endpoint credentials are exported verbatim, as the dashboard already shows them.

UI

• Header buttons Export (downloads <name>-configurations.json via a small download.js helper) and Import (file picker).
• Import dialog previews each configuration with an editable name, flags "Already exists" / "Duplicate name" / "Name required", and keeps Import disabled until every row is clean. Warns when the export comes from a different data store type.

Tests

DataStoreTransferTests covers the export shape (no connection string, no runtime state, sorted tables), create with and without endpoints, name clash refused with target untouched, import under a new name beside the original, duplicate names in one document, invalid table values, non-export files, and unauthenticated access. Full suite: 43 passed.