Standardize table naming convention to snake_case

[bartmichalak]

Currently, database tables use lowercase naming without separators (e.g., userconnection), which reduces readability. We should migrate to snake_case convention (e.g., user_connection or user_connections) to improve code clarity and align with common Python/SQL standards.

My preference and the standard in frameworks like Django and Ruby on Rails is snake_case with plural form (e.g., user_connections).