Fix SQLite concurrency for refresh rotation tests

[shreyansh-tech21]

What does this PR do?

Improves SQLite behavior for concurrent refresh-token rotation and hardens the refresh rotation tests so they match real multi-threaded use: workers run with their own Flask app_context, tests use a shared file-backed SQLite DB (not isolated :memory: per connection), and the app configures SQLAlchemy’s BEGIN IMMEDIATE pattern on SQLite so two threads cannot both “win” the same rotation race. The concurrent test still asserts exactly one successful rotate_refresh_token and one RefreshRotationError (reuse).

Related Issue

Closes #12

Changes made

• Add sqlite_db.py with configure_sqlite_immediate_transactions() (connect → isolation_level = None, begin → BEGIN IMMEDIATE) for SQLite engines only.
• Call that configuration from app.py immediately after db.init_app(app) (inside app_context()), so production and tests using SQLite get serialized writer behavior without affecting Postgres.
• Update tests/test_refresh_rotation.py: tmp_path file SQLite URI + db.engine.dispose() in the fixture; per-thread with app.app_context() in concurrent workers; docstring describing threading + SQLite safety; stricter assertions on the loser (reuse).

Checklist

• I have tested my changes
• My code follows project guidelines
• No new errors introduced

[github-actions]

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[neeru24]

@shreyansh-tech21 resolve conflicts

[shreyansh-tech21]

Hi @neeru24 and @iarmaanx , I have resolved all the conflicts. Made code changes as we per the proposal. Please let me know if there are any further code changes from my side. Thank you!