This commit standardizes the naming of identifier and API key fields
across the codebase to use consistent camel case (e.g., `ID`, `APIKey`,
`DatabaseURL`). This includes updates to struct fields, method names,
function parameters, and environment variable references. The changes
improve code clarity and maintainability by reducing ambiguity and
aligning with Go naming conventions. No functional behavior is changed.
Introduce slog-based structured logging throughout the booking service
and
server handlers. Add configurable log level via LOG_LEVEL environment
variable. Replace legacy log usage with slog and propagate logger to
booking service for improved observability.
Update the documentation for the Config struct to clarify that environment variables must be prefixed, e.g., with "APP_", to be recognized. This change improves clarity for users configuring the application.
