Add MariaDB database support for storing transaction data

Introduces a new database layer to persist CSV transaction data in MariaDB,
enabling both invoicing and sales reporting queries. This replaces the
previous file-to-file-only processing.

Changes:
- Add sqlx, tokio, toml, anyhow, bigdecimal dependencies to Cargo.toml
- Create config module for TOML-based configuration (database credentials)
- Create db module with connection pool, models, and repository
- Create commands module with 'import' subcommand for CSV ingestion
- Refactor main.rs to use subcommand architecture (import/generate)
- Add migration SQL file for manual database schema creation

Schema (3 tables):
- customers: customer_number, card_report_group (1=fleet, 3/4=retail)
- cards: card_number, card_type, customer_id (nullable for anonymous)
- transactions: full transaction data with FK to cards/customers

Usage:
  cargo run -- import <csv-file>   # Import to database
  cargo run -- generate <csv> <dir>  # Generate HTML invoices (unchanged)

Configuration:
  cp config.example.toml config.toml  # Edit with database credentials
  mysql < migrations/001_initial_schema.sql  # Create database first

config.example.toml

@@ -0,0 +1,6 @@
+[database]
+host = "localhost"
+port = 3306
+user = "root"
+password = ""
+name = "rusty_petroleum"