fix: resource leaks and connection safety

[0xNyk]

Summary

• Use with context managers for all SQLite connections in simulation.py, simulation_runner.py, and simulation scripts
• Add @after_this_request temp file cleanup in report download endpoint
• Add finally block for file handle safety in simulation runner subprocess creation

Test plan

• Verify backend starts: cd backend && uv run python -c "from app import create_app; create_app()"
• Test report download endpoint creates and cleans up temp files
• Verify simulation scripts handle SQLite connections properly

[0xNyk]

Verification

All checks pass on merged code:

Test	Result
with sqlite3.connect in simulation.py (2 instances)	✅ PASS
with sqlite3.connect in simulation_runner.py (1 instance)	✅ PASS
with sqlite3.connect in run_parallel_simulation.py (2 instances)	✅ PASS
with sqlite3.connect in run_twitter_simulation.py (1 instance)	✅ PASS
with sqlite3.connect in run_reddit_simulation.py (1 instance)	✅ PASS
@after_this_request temp file cleanup in report.py	✅ PASS
create_app() succeeds	✅ PASS