feat: per-channel message list for Solid Cable

[eclectic-coding]

Closes #33

Summary

• Adds GET /cable/channels/:channel_hash — paginated, reverse-chronological list of SolidCable::Message records for a specific channel
• Each row shows message ID, payload preview (truncated to 120 chars), and relative sent time with exact timestamp on hover
• Channel names in the channel browser (GET /cable) are now links using channel_hash as the URL-safe identifier, avoiding encoding issues with JSON-style channel strings
• ROADMAP updated to remove shipped "Channel browser" item; README Solid Cable section updated to reflect current feature set

Test plan

• Seed cable messages (bundle exec rake dev:seed) and visit /cable — channel names should be clickable links
• Click a channel — should open the message list with paginated rows
• Verify payload preview truncates long payloads at 120 chars with ellipsis
• Verify hover on sent-time cell shows exact timestamp
• Visit /cable/channels/<unknown_hash> — should render empty state
• bundle exec rake passes clean

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.29%. Comparing base (c24e34f) to head (62ae75e).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #60      +/-   ##
==========================================
+ Coverage   99.10%   99.29%   +0.18%     
==========================================
  Files          32       33       +1     
  Lines         559      565       +6     
==========================================
+ Hits          554      561       +7     
+ Misses          5        4       -1     

Files with missing lines	Coverage Δ
...pp/controllers/solid_stack_web/cable_controller.rb	100.00% <100.00%> (ø)
...llers/solid_stack_web/cable_messages_controller.rb	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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