Add multi-environment support for database configuration

Introduces separate databases and config files for dev, test, and prod
environments. The application now defaults to production, with --env flag
to specify alternative environments.

Changes:
- Update config.rs to support env-based loading (config.toml -> config.<env>.toml -> config.example.toml)
- Add Env enum (Prod, Dev, Test) with database name mapping
- Add --env flag to CLI commands (defaults to prod)
- Add 'db setup' command to create database and schema
- Split migrations into env-specific database creation and shared schema
- Update .gitignore to track config.example.toml but ignore config.toml and config.<env>.toml files
- Update config.example.toml as a template with placeholder values
- Delete 001_initial_schema.sql, replaced by 002_schema.sql + env-specific files

Config loading order:
  1. config.toml (local override)
  2. config.<env>.toml (environment-specific)
  3. config.example.toml (fallback)

Database names:
  - prod: rusty_petroleum
  - dev:  rusty_petroleum_dev
  - test: rusty_petroleum_test

Usage:
  cargo run -- db setup --env dev       # Setup dev database
  cargo run -- import data.csv --env dev # Import to dev
  cargo run -- db setup                # Setup prod (default)
  cargo run -- import data.csv         # Import to prod (default)

migrations/002_schema.sql

@@ -0,0 +1,49 @@
+-- Schema for rusty_petroleum
+-- Run after creating the database
+
+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;