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/001_dev.sql

@@ -0,0 +1,2 @@
+-- Create development database
+CREATE DATABASE IF NOT EXISTS rusty_petroleum_dev;

migrations/001_prod.sql

@@ -0,0 +1,2 @@
+-- Create production database
+CREATE DATABASE IF NOT EXISTS rusty_petroleum;

migrations/001_test.sql

@@ -0,0 +1,2 @@
+-- Create test database
+CREATE DATABASE IF NOT EXISTS rusty_petroleum_test;

migrations/002_schema.sql

@@ -1,8 +1,5 @@
--- Initial schema for rusty-petroleum
--- Run this against your MariaDB database before importing data
-
-CREATE DATABASE IF NOT EXISTS rusty_petroleum;
-USE rusty_petroleum;
+-- Schema for rusty_petroleum
+-- Run after creating the database
 
 CREATE TABLE IF NOT EXISTS customers (
     id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,