Add a count preview to maintenance task UI

[mikee649]

Add a count preview to maintenance task UI

Summary

• Displays an estimated count of items to be processed before running a task, giving operators visibility into the scope of a task run
• Fetches the count asynchronously via a new JSON endpoint so the main page load is not blocked
• Automatically re-fetches the count when task parameters change, keeping the estimate in sync with the current input

Motivation

When running a maintenance task, operators had no way to gauge how many items would be processed beforehand. This made it difficult to assess the impact of a run or verify that parameters were configured correctly. A count preview solves this by showing a live estimate directly on the task show page.

What changed

New count endpoint (GET /tasks/:id/count)

• Added a count action to TasksController that returns { count: <integer|null> } as JSON.
• Errors are gracefully rescued, returning { count: null } so the UI can hide the indicator instead of showing a broken state.
• Route added as a member action on the tasks resource.

TaskDataShow#count method

• Computes the estimated item count by first trying the task's #count method, then falling back to collection.count.
• Returns nil for CSV tasks (collection depends on uploaded file content), deleted tasks, and when the query times out.
• Wrapped in a 15-second Timeout to prevent long-running count queries from blocking the endpoint.

TaskDataShow#no_collection? method

• New predicate to detect tasks that have no collection (e.g. one-off tasks). These tasks skip the count preview since the indicator is not meaningful.

Frontend (show.html.erb)

• Renders a #task-count div that starts with "Estimating expected items count..." placeholder text.
• Inline <script> fetches the count on page load and displays it as "N items expected to be processed".
• Listens on focusout and change events on the task form to re-fetch when parameters are modified.
• Hidden for deleted tasks, CSV tasks, and no-collection tasks.
• CSP updated with a new script hash for the inline script.

How it looks

• Task with collection: Shows "42 items expected to be processed" below the parameters form.
• Task with parameters: Count updates dynamically as parameters change (e.g. entering post IDs narrows the count).
• CSV / no-collection / deleted tasks: Count indicator is not shown.
• Timeout or error: Count indicator is silently hidden.