Add collection download as JSON endpoint

[brandonjackson]

Description

This PR adds a new endpoint to download collection items as JSON files. Users can now export collection data from a project via GET /download/collection with query parameters for project_id and collection name.

The implementation includes:

• New download_collection_json/2 controller action with proper authorization checks
• Streaming response handler (stream_collection_items/2) that efficiently processes large collections in batches of 500 items
• Download button in the collections UI component
• Comprehensive test coverage for success cases, authorization, and error scenarios

Validation steps

1. Navigate to a project with collections
2. Click the "Download" button next to a collection
3. Verify the JSON file downloads with correct filename and content
4. Test with empty collections (should return [])
5. Verify authorization: non-project members should receive 401, non-existent collections should return 404
6. Run test suite: mix test test/lightning_web/controllers/download_controller_test.exs

Additional notes for the reviewer

• Authorization is enforced via ProjectUsers and Permissions.can/4 to ensure only project members can download
• Collection ownership is validated by checking collection.project_id == project.id
• Streaming implementation uses Plug.Conn.chunk/2 to handle large datasets efficiently
• The endpoint follows the same pattern as the existing download_project_yaml action

AI Usage

• I have used Claude Code
• I have used another model
• I have not used AI

Pre-submission checklist

• I have performed an AI review of my code (we recommend using /review with Claude Code)
• I have implemented and tested all related authorization policies. (e.g., :owner, :admin, :editor, :viewer)
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR

https://claude.ai/code/session_0133om1iWqftcidvyYshXDFQ

[codecov]

Codecov Report

❌ Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.55%. Comparing base (75489ef) to head (f84a047).

Files with missing lines	Patch %	Lines
.../lightning_web/controllers/downloads_controller.ex	95.65%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4509   +/-   ##
=======================================
  Coverage   89.55%   89.55%           
=======================================
  Files         425      425           
  Lines       20295    20320   +25     
=======================================
+ Hits        18175    18198   +23     
- Misses       2120     2122    +2     

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[midigofrank]

@brandonjackson , this is definitely in the right direction. We can polish it a little bit to reduce duplication of the streaming logic and also to reuse the auth logic used by other actions. 🤔 I'd be curious to see how the app handles a 50mb collection