Add SMTP authentication failure blocking system with admin dashboard

[lucaforni]

This commit implements a comprehensive system to protect against brute force attacks on SMTP authentication by automatically blocking IPs after repeated failed authentication attempts.

Features:

• Automatic IP blocking after X failed auth attempts (default: 5, configurable)
• Configurable block duration Y minutes (default: 120, configurable)
• Blocks persist across all SMTP auth methods (PLAIN, LOGIN, CRAM-MD5)
• Counter resets automatically on successful authentication
• Admin web dashboard to view, search, and manage blocked IPs
• Manual block/unblock capabilities for individual or all IPs
• Prometheus metrics integration for monitoring
• Detailed logging for security auditing

Technical Implementation:

• AuthFailureTracker class manages blocking logic using Rails.cache
• SHA-256 hashed cache keys for security
• Efficient indexing system for listing blocked IPs
• RESTful admin routes for IP management
• HAML-based responsive UI integrated with existing admin interface

Testing:

• 39 unit tests for AuthFailureTracker
• 14 integration tests for SMTP client auth blocking
• 7 controller tests for admin dashboard
• All 60 new tests passing

Configuration (via postal.yml or environment variables):

• smtp_server.auth_failure_threshold (default: 5)
• smtp_server.auth_failure_block_duration (default: 120 minutes)

Documentation:

• Complete user guide (doc/SMTP_AUTH_BLOCKING.md)
• Configuration examples for various security levels
• Technical documentation for developers
• Updated main configuration documentation

Files Modified:

• app/lib/smtp_server/client.rb - Integrated blocking checks
• app/views/layouts/application.html.haml - Added menu link
• config/routes.rb - Added admin routes
• lib/postal/config_schema.rb - Added configuration options
• doc/config/configuration.md - Documented new feature

Files Created:

• app/lib/smtp_server/auth_failure_tracker.rb (323 lines)
• app/controllers/admin_blocked_ips_controller.rb (74 lines)
• app/views/admin_blocked_ips/index.html.haml (118 lines)
• 3 comprehensive test files (520+ lines)
• 3 documentation files