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.
### TL;DR
Enhanced invoice generation with improved formatting and Euro symbol display
### What changed?
- Added Euro symbol (€) to monetary values in the invoice template
- Implemented new invoice data structure with dedicated types for lines and payments
- Created ToInvoice method to properly format booking data for invoice generation
- Added HTML template parsing and rendering functionality
- Improved date formatting for consistency
- Added new API endpoint for booking creation
### How to test?
1. Create a new booking through the API
2. Navigate to the PDF generation endpoint
3. Verify that monetary values display with Euro symbol
4. Check that dates are properly formatted
5. Confirm that payment history and totals are correctly calculated
6. Validate that the generated HTML maintains proper formatting
### Why make this change?
To improve invoice readability and consistency by standardizing monetary value display and providing better data structure for invoice generation. This change also makes the system more maintainable by separating concerns between data transformation and presentation.
