Add exception handling for DB transactions

[harsh-kumar-patwa]

Summary

Right now, if a database commit or delete fails in any of the CRUD functions, there's no rollback — the SQLAlchemy session is left in a broken state. The API endpoints also let these errors bubble up as raw 500s, and the chat endpoints crash instead of falling back to in-memory history.

This PR fixes that.

Changes

• crud.py — wrapped all 5 write functions (create_conversation, update_conversation_title, delete_conversation, create_message, delete_message) with try/except so they call db.rollback() on failure before re-raising
• conversations.py — CRUD endpoints now catch DB errors and return proper HTTP 500 responses. Chat endpoints use a db_persist flag so if the DB call fails, they fall back to in-memory history instead of crashing. get_history_str also handles DB read failures gracefully.
• test_database_crud.py — added 5 tests to verify rollback is called when a commit fails

Context

Resolves #185

All existing tests pass (54/54), no lint issues.

[Copilot]

Pull request overview

This PR improves resilience around SQLAlchemy session failures by ensuring failed write operations roll back transactions and by adding API-layer handling for database exceptions, including a fallback for chat history persistence.

Changes:

• Add try/except + db.rollback() around DB write operations in crud.py (and log before re-raising).
• Catch SQLAlchemyError in conversation CRUD endpoints and convert them into explicit HTTP 500 responses.
• Add tests asserting rollback() is invoked when commit() fails in each write CRUD function.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
backend/src/database/crud.py	Wraps write operations with rollback + logging on SQLAlchemyError.
backend/src/api/routers/conversations.py	Adds DB exception handling for CRUD endpoints and introduces db_persist fallback logic for chat persistence.
backend/tests/test_database_crud.py	Adds unit tests to verify rollback behavior on commit failures.

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