Add routes, controllers, and views for dashboard, queues, jobs, and failed jobs

[eclectic-coding]

Summary

• Defines engine routes: dashboard root, queues (pause/resume), jobs (status/queue filter), failed jobs (retry/discard/discard-all)
• Adds ApplicationController with auth hook and Pagy::Backend; ApplicationHelper with Pagy::Frontend
• Implements DashboardController, QueuesController, JobsController, and FailedJobsController
• Builds views for all four pages with stat cards, filterable job tables, and action buttons
• Adds a minimal CSS stylesheet with badges, stat cards, table styles, and pagy pagination styles
• Adds solid_queue and pagy as runtime dependencies

Correctness notes

• SolidQueue::Queue is a PORO (not ActiveRecord) — uses pagy_array and .sort_by instead of AR scopes
• discard_all does not exist on FailedExecution; corrected to discard_all_in_batches
• Queue count on the dashboard uses SolidQueue::Job.select(:queue_name).distinct.count

Test plan

• Mount the engine in a host app (mount SolidQueueDashboard::Engine, at: "/jobs") and visit /jobs
• Verify dashboard stat cards reflect live queue counts
• Verify jobs page filters by status (ready / scheduled / running / blocked / failed)
• Click a queue name link to verify per-queue job filtering
• Pause and resume a queue; confirm badge updates
• Enqueue a failing job; confirm it appears on Failed Jobs page
• Retry a failed job; confirm it re-enters the ready queue
• Discard a single failed job and confirm removal
• Use "Discard All" and confirm all failed jobs are removed
• Configure SolidQueueDashboard.authenticate block and verify unauthorized access is blocked

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