feat: include photo result summary counts in activity response

[siculo]

Summary

Add photo result summary counts to the GET /api/activities/{id}
response so that callers can quickly determine how many photos were
processed successfully and how many produced no content, without
having to fetch the full results endpoint.

Motivation

The results endpoint already exposes per-photo status, including
photos for which no content was generated. However, GET /api/activities/{id} returns no aggregated counts, forcing clients
to either call the results endpoint or display no summary at all.

Adding counts to the activity response enables the plugin to show
at a glance — directly in the activity list — how many photos
completed successfully and how many had no output (e.g.,
"522 processed · 23 without content").

Scope

API

• Add a photo_summary object to the GET /api/activities/{id}
  response body:

  "photo_summary": {
    "total": 522,
    "completed": 499,
    "without_content": 23
  }

• Compute counts from the existing per-photo result data; no new
  storage required
• Return null for photo_summary if the activity has not yet
  started processing

Plugin

• Display the summary counts in the activity list item and/or the
  activity detail view, consistent with issue feat: enrich activity list item with additional metadata #121
• Show without_content count only when greater than zero

Documentation

• Update the API reference for GET /api/activities/{id}

Acceptance criteria

• GET /api/activities/{id} returns photo_summary with
  total, completed, and without_content
• Counts match the per-photo data returned by the results endpoint
• photo_summary is null for activities not yet started
• Plugin displays the counts in the activity list
• Existing activity responses without the field are not broken