DEV: Refactor uploads_importer script (#29292)

* DEV: Implement uploads command entrypoint

- Setup Thor UploadsCommand for CLI
- First pass at modularizing various parts of the exising `uploads_import` script

* DEV: First attempt at modularizing missing uploads fixer task

Move missing upload fix to a dedicated uploads task implementation unit

* DEV: First attempt at modularizing missing uploads uploader task

Move uploader to a dedicated uploads task implementation unit

* DEV: First attempt at modularizing missing uploads optimizer task

Move optimizer to a dedicated uploads task implementation unit

* DEV: Various follow up fixes to get optimization working

- Start threads early
- Improve "log" message formatting
- Add missing `copy_to_tempfile` method on "uploader" task

* DEV: Refactor a bit more

Deduplicate and move most of threading premitives to base task as-is

* DEV: Remove redundant condition in uploads db migration

* DEV: More deduplication

Move task retry logic to base class and tidy up other implementation
details carried over from the existing script

migrations/lib/settings_parser.rb

@@ -0,0 +1,81 @@
+# frozen_string_literal: true
+
+module Migrations
+  class SettingsParser
+    class InvalidYaml < StandardError
+    end
+    class ValidationError < StandardError
+    end
+
+    REQUIRED_KEYS = %i[source_db_path output_db_path root_paths]
+
+    def initialize(options)
+      @options = options
+
+      validate!
+    end
+
+    def [](key)
+      @options[key]
+    end
+
+    def []=(key, value)
+      @options[key] = value
+    end
+
+    def fetch(key, default)
+      @options.fetch(key, default)
+    end
+
+    def self.parse!(path)
+      new(YAML.load_file(path, symbolize_names: true))
+    rescue Psych::SyntaxError => e
+      raise InvalidYaml.new(e.message)
+    end
+
+    private
+
+    def validate!
+      validate_required_keys
+      validate_paths
+      validate_options
+    end
+
+    def validate_required_keys
+      missing = REQUIRED_KEYS - @options.keys
+
+      raise ValidationError, "Missing required keys: #{missing.join(", ")}" if missing.any?
+    end
+
+    def validate_paths
+      %i[source_db_path output_db_path].each do |key|
+        path = @options[key]
+
+        next unless path
+
+        dir = File.dirname(path)
+        raise ValidationError, "Directory not writable: #{dir}" unless File.writable?(dir)
+      end
+
+      if !@options[:root_paths].is_a?(Array)
+        raise ValidationError, "Root paths must be an array of paths"
+      end
+
+      @options[:root_paths].each do |path|
+        raise ValidationError, "Directory not readable: #{path}" unless File.readable?(path)
+      end
+    end
+
+    def validate_options
+      validate_thread_count_factor if @options[:thread_count_factor]
+    end
+
+    def validate_thread_count_factor
+      count = @options[:thread_count_factor]
+
+      unless count.is_a?(Numeric) && count.positive?
+        raise ValidationError, "Thread count factor must be numeric and positive"
+      end
+    end
+  end
+end