fix(database): allow nullable resource_id in InMemory and SQLite backends

[h0ll0w-AkuZr0guY]

📝 Pull Request Summary

This PR makes the resource_id parameter optional (str | None = None) in the InMemory and SQLite database backends, aligning their signatures with the PostgreSQL backend and resolving a runtime TypeError.

---

✅ What does this PR do?

• Updates the MemoryItemRepo protocol in src/memu/database/repositories/memory_item.py to allow resource_id: str | None = None.
• Updates create_item and create_item_reinforce methods in the InMemory backend (inmemory/repositories/memory_item_repo.py).
• Updates create_item and create_item_reinforce methods in the SQLite backend (sqlite/repositories/memory_item_repo.py).

---

🤔 Why is this change needed?

• Context/Impact: When users call the standard MemoryService.create_memory_item() API, it does not provide a resource_id. Previously, this caused a TypeError (missing 1 required keyword-only argument: 'resource_id') when using InMemory or SQLite backends.
• Consistency: The PostgreSQL backend was already fixed to make this optional in PR fix: allow nullable resource_id in Postgres items #232. This PR extends that fix to the remaining backends.
• Fixes Bug: InMemory/SQLite backends fail with missing resource_id #318

---

🔍 Type of Change

Please check what applies:

• Bug fix
• New feature
• Documentation update
• Refactor / cleanup
• Other (please explain)

---

✅ PR Quality Checklist

• PR title follows the conventional format (feat:, fix:, docs:)
• Changes are limited in scope and easy to review
• Documentation updated where applicable
• No breaking changes (or clearly documented)
• Related issues or discussions linked

---

📌 Optional

• Screenshots or examples added (if applicable)
• Edge cases considered
• Follow-up tasks mentioned