Add db_session_middleware to main.py that opens the connection at the
start of each request and closes it in a try/finally block, ensuring
connections are always returned even on uncaught exceptions.
Remove all individual db.close() calls from 30 router files in
app/routers_v2/ — the middleware now handles all code paths.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
