#2675 Add JSON response format for full SDK/CLI coverage

Summary

Add JSON response formats across the API surface so the SDK (fizzy-sdk#9) and CLI can call every operation.

• Add JSON respond_to blocks to 14 void-response controller actions (involvements, entropies, column reordering, readings, publishes, roles, avatars, settings, join codes, push subscriptions)
• Add steps index endpoint (GET /cards/:id/steps.json)
• Add JSON views for paginated card lists (stream, not-now, closed columns)
• Add JSON search with distinct-card pagination — paginates Card records via Card.mentioning + .distinct rather than deduping Search::Record objects after pagination
• Add JSON views for settings/config endpoints (account settings, join codes, notification settings)
• Add JSON response format to data exports with status polling (create returns 201 with id/status; show returns any-status export for JSON)
• Extend access token JSON API (index view, destroy format, add id + created_at to create response)
• Push subscriptions distinguish new (201) vs existing (200) via previously_new_record?

Test plan

• bin/rails test — 1167 tests, 0 failures
• Verify paginated endpoints return X-Total-Count and Link headers
• Verify 422 for join code update with invalid data
• Verify 404 for missing/expired export
• Verify search dedup: query matching both card title and comment returns 1 card in JSON