Move multi-DB operations from WordService into WordRepository

[Copilot]

Part of coordinating atomic updates to FrontierCollection and WordsCollection via transactions. Goal 2 of the PR: move all multi-collection database logic out of the service layer and into new WordRepository methods (which will be wrapped in transactions in Goal 4).

New IWordRepository / WordRepository methods

// Word replacement: insert newWord into Words+Frontier, delete oldWordId from Frontier
Task<Word> CreateAndDeleteFrontier(Word newWord, string oldWordId);

// Soft-delete: insert deletedWord into Words only, delete wordId from Frontier
Task<Word> AddAndDeleteFrontier(Word deletedWord, string wordId);

WordService refactors

• Update: replaced Create(userId, word) + DeleteFrontier(oldWordId) with _wordRepo.CreateAndDeleteFrontier(PrepEditedData(userId, word), oldWordId)
• DeleteFrontierWord: replaced Add(userId, word) + DeleteFrontier(wordId) with _wordRepo.AddAndDeleteFrontier(PrepEditedData(userId, word), wordId)
• Removed now-unused private WordService.Add(string userId, Word word) helper

Scope

MergeService needs no direct changes — it delegates to the refactored WordService methods. WordRepositoryMock has mock implementations of both new methods. Wrapping the new WordRepository methods in transactions is deferred to Goal 4.

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This change is 

[imnasnainaec]

@copilot Execute the plan.

[Copilot]

@copilot Execute the plan.

Done in commit 0e7b6e0. Changes made:

• IWordRepository: Added CreateAndDeleteFrontier(Word newWord, string oldWordId) and AddAndDeleteFrontier(Word deletedWord, string wordId)
• WordRepository: Implemented both — each encapsulates the two DB ops that were previously split across the service layer (ready to be wrapped in a transaction in Goal 4)
• WordService.Update: replaced Create(userId, word) + DeleteFrontier(...) with _wordRepo.CreateAndDeleteFrontier(PrepEditedData(userId, word), oldWordId)
• WordService.DeleteFrontierWord: replaced Add(userId, word) + DeleteFrontier(...) with _wordRepo.AddAndDeleteFrontier(PrepEditedData(userId, word), wordId)
• WordService: removed the now-unused private Add(string userId, Word word) helper
• WordRepositoryMock: added mock implementations of both new methods

All 46 existing tests pass unchanged.