Refactor booking retrieval to return errors instead of nil values,
enabling more robust error handling throughout the booking, payment,
and PDF endpoints. Add custom HTTP error page rendering for not found
and internal server errors. Update interfaces and tests to match new
method signatures. This improves user feedback and code maintainability.
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.
