Favicon Infrastructure: Config, Model, Job, and Discovery

What Was Built

• FaviconsSettings configuration class with enabled, fetch_timeout, max_download_size, retry_cooldown_days, allowed_content_types
• Source model has_one_attached :favicon with if defined?(ActiveStorage) guard
• Favicons::Discoverer implementing /favicon.ico -> HTML link tag parsing -> Google Favicon API cascade
• FaviconFetchJob with early-return guards (missing source, disabled, blank URL, already attached, cooldown)
• Favicons module autoload wired in lib/source_monitor.rb, FaviconsSettings require in configuration.rb

Files Modified

• lib/source_monitor/configuration/favicons_settings.rb -- new settings class with defaults and enabled?
• lib/source_monitor/configuration.rb -- added favicons_settings require, attr_reader, initialization
• app/models/source_monitor/source.rb -- has_one_attached :favicon with ActiveStorage guard
• lib/source_monitor/favicons/discoverer.rb -- multi-strategy favicon discovery with HTML parsing and validation
• app/jobs/source_monitor/favicon_fetch_job.rb -- background job with cooldown tracking via metadata JSONB
• lib/source_monitor.rb -- Favicons module autoload declaration
• test/lib/source_monitor/configuration/favicons_settings_test.rb -- 13 tests for settings
• test/models/source_monitor/source_favicon_test.rb -- 3 tests for model attachment
• test/lib/source_monitor/favicons/discoverer_test.rb -- 16 tests for discovery cascade
• test/jobs/source_monitor/favicon_fetch_job_test.rb -- 10 tests for job guards and behavior