Add multi-database support

[antarr]

Summary

• Auto-detect MySQL/MariaDB databases from Rails database.yml (supports Rails 5.2-8.1)
• Per-database settings via config/mysql_genius.yml with environment-specific overrides
• Database switcher dropdown in the dashboard header (hidden in single-DB setups)
• URL-scoped routing: /mysql_genius/analytics/, /mysql_genius/primary/execute, etc.
• config.database(:name) Ruby DSL for per-database overrides in the initializer
• Full backward compatibility: single-database setups work without any changes

Per-database configurable settings

blocked_tables, masked_column_patterns, featured_tables, default_columns, max_row_limit, default_row_limit, query_timeout_ms

Architecture

• DatabaseConfig -- per-database settings with fallback to global Configuration
• DatabaseRegistry -- YAML loading, auto-detection from Rails configs, version-aware (5.2-8.1)
• BaseController -- resolve_database! before_action, connection helper replacing 25+ hardcoded ActiveRecord::Base.connection calls
• AI services -- accept connection: keyword so queries run against the correct database

New files

File	Purpose
lib/mysql_genius/database_config.rb	Per-database settings with fallback
lib/mysql_genius/database_registry.rb	YAML loading & auto-detection
spec/mysql_genius/database_config_spec.rb	8 examples
spec/mysql_genius/database_registry_spec.rb	11 examples

Test plan

• 94 examples, 0 failures
• RuboCop clean (0 offenses)
• Test with a multi-database Rails app (primary + analytics)
• Verify single-database backward compat (no YAML, existing setup)
• Verify database switcher dropdown appears only in multi-DB mode
• Verify URL routing: /mysql_genius/, /mysql_genius/primary/, /mysql_genius/analytics/
• Verify YAML config loading with environment override

[antarr]

Closing — superseded by Phase 2c desktop profile switching. The Rails adapter uses the host app's ActiveRecord connection and doesn't need its own multi-database management.