Add invoice generator for fuel station transactions

- Read CSV files from input/ directory
- Generate static HTML invoices grouped by customer and card
- Filter transactions to only include fleet customers
- Compact print-friendly layout with 2 decimal precision

Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "invoice-generator"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+askama = "0.15.5"
+chrono = "0.4.44"
+csv = "1.4.0"
+serde = "1.0.228"