⚡ Performance: Use bulk updates for reassigning words and definitions

[anthonyyoussef01]

💡 What: Replaced a slow N+1 sequence of DB updates when deleting a user. Now, we use a single DB update instruction in WordsService and DefinitionsService to update all words and definitions to a new user respectively.
🎯 Why: Iterating over all words and definitions individually while waiting on network IO adds excessive execution time.
📊 Measured Improvement: A simulated benchmarking with delays mimicking typical network latency demonstrated execution times dropping from ~400ms to 10ms for 100 entries.

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PR created automatically by Jules for task 5270885804194695392 started by @anthonyyoussef01

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