Add channel proxy plug with endpoint routing

[stuartc]

Description

This PR adds the channel proxy plug that routes incoming HTTP requests to channel sink URLs via the Weir streaming proxy.

• Adds ChannelProxyPlug mounted at /channels/:channel_id in the endpoint
• Validates sink_url on channel changeset (must be absolute HTTP/HTTPS URL)
• Uses Weir for streaming HTTP proxy with configurable adapter
• Adds weir dependency (GitHub source, with WEIR_PATH local override)
• Path traversal protection and comprehensive test coverage

Closes #4401

Validation steps

1. Run mix test test/lightning_web/plugs/channel_proxy_plug_test.exs
2. Run mix test test/lightning/channels_test.exs

Additional notes for the reviewer

1. Weir is sourced from GitHub (not yet on Hex). Set WEIR_PATH env var for local development.

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
• I have implemented and tested all related authorization policies.
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.31%. Comparing base (fd6c24e) to head (f42efd9).
⚠️ Report is 1 commits behind head on channels.

Additional details and impacted files

@@            Coverage Diff            @@
##           channels    #4418   +/-   ##
=========================================
  Coverage     89.31%   89.31%           
=========================================
  Files           430      431    +1     
  Lines         20103    20121   +18     
=========================================
+ Hits          17955    17972   +17     
- Misses         2148     2149    +1     

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