Reject PUT changes to ImportList.MinRefreshInterval instead of discarding them

[jbob06]

Summary

Fixes #23.

ImportListDefinition.MinRefreshInterval is a fixed, per-list-type constant declared on each IImportList implementation (e.g. GoodreadsBookshelf => TimeSpan.FromHours(12)), intentionally excluded from persistence (TableMapping.cs has .Ignore(i => i.MinRefreshInterval)), and unconditionally overwritten from the provider on every load via ImportListFactory.SetProviderCharacteristics. So a PUT /api/v1/importlist/{id} that changed it was silently accepted (HTTP 202), with the change discarded - the response goes on to echo the provider's value, not the one that was requested.

This adds a PutValidator rule comparing the requested value against the provider's authoritative value (via _importListFactory.Find(id) + SetProviderCharacteristics) and rejects the PUT with a clear validation error when they differ, instead of silently discarding it.

Two things to flag:

• TimeSpan.Zero is always allowed through, since MinRefreshInterval is a non-nullable TimeSpan and a client that omits the field from its request body gets zero on deserialization. No provider ever uses zero, so this can't mask a real attempt to change the value, and it avoids a spurious 400 for clients that only send the fields they mean to change.
• An unknown id is left to the normal Get()-based 404 handling rather than surfacing as a validation error from this rule.
• POST isn't covered here (as written it couldn't be - Find(0) returns null for a not-yet-created list) - ImportList minRefreshInterval cannot be changed via the API (PUT is silently ignored) #23 is specifically about PUT, and POST already discards a client-supplied value the same way it always has.

Test plan

• New unit tests in ImportListControllerMinRefreshIntervalFixture (4 cases: reject a real change, allow leaving it unchanged, allow omitting the field, defer to normal not-found handling for an unknown id) - built against the real ImportListController and the same ValidateResource method the ASP.NET pipeline calls on every PUT
• Full existing test suite passes unchanged (2856/2856)