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

migrations/001_initial_schema.sql

@@ -0,0 +1,52 @@
+-- Initial schema for rusty-petroleum
+-- Run this against your MariaDB database before importing data
+
+CREATE DATABASE IF NOT EXISTS rusty_petroleum;
+USE rusty_petroleum;
+
+CREATE TABLE IF NOT EXISTS customers (
+    id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
+    customer_number VARCHAR(50) NOT NULL UNIQUE,
+    card_report_group TINYINT UNSIGNED NOT NULL DEFAULT 0,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+    INDEX idx_customer_number (customer_number)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+
+CREATE TABLE IF NOT EXISTS cards (
+    id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
+    card_number VARCHAR(50) NOT NULL UNIQUE,
+    card_type VARCHAR(50),
+    customer_id INT UNSIGNED NULL,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+    INDEX idx_card_number (card_number),
+    INDEX idx_customer_id (customer_id),
+    FOREIGN KEY (customer_id) REFERENCES customers(id) ON DELETE SET NULL
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+
+CREATE TABLE IF NOT EXISTS transactions (
+    id BIGINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
+    transaction_date DATETIME NOT NULL,
+    batch_number VARCHAR(20) NOT NULL,
+    amount DECIMAL(10,2) NOT NULL,
+    volume DECIMAL(10,3) NOT NULL,
+    price DECIMAL(8,4) NOT NULL,
+    quality_code INT NOT NULL,
+    quality_name VARCHAR(50) NOT NULL,
+    station VARCHAR(20) NOT NULL,
+    terminal VARCHAR(10) NOT NULL,
+    pump VARCHAR(10) NOT NULL,
+    receipt VARCHAR(20) NOT NULL,
+    control_number VARCHAR(20),
+    card_id INT UNSIGNED NOT NULL,
+    customer_id INT UNSIGNED NULL,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    INDEX idx_transaction_date (transaction_date),
+    INDEX idx_batch_number (batch_number),
+    INDEX idx_card_id (card_id),
+    INDEX idx_customer_id (customer_id),
+    INDEX idx_station (station),
+    FOREIGN KEY (card_id) REFERENCES cards(id),
+    FOREIGN KEY (customer_id) REFERENCES customers(id) ON DELETE SET NULL
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;