Add sql() function for database queries in templates

[vloothuis]

Summary

• Add sql() function to templates for querying SQL databases (MySQL, PostgreSQL, SQLite)
• Add optional sqlalchemy>=2.0 dependency (pip install mdcomp[sql])
• Add --db-url CLI option and MDCOMP_DB_URL env var support
• Cache database engines to prevent connection pool exhaustion
• Sanitize URLs in error messages to prevent credential leakage

Usage

{% for user in sql("SELECT name, email FROM users WHERE status = :status", {"status": "active"}) %}
- {{ user.name }} ({{ user.email }})
{% endfor %}

Test plan

• Unit tests for run_sql() function (parameterized queries, edge cases)
• Integration tests for sql() in templates
• Tests for URL sanitization (credential hiding)
• Tests for engine caching
• Manual verification with SQLite