feat: cache entry detail page

[eclectic-coding]

Summary

• Adds GET /cache/entries/:id — shows full key, byte size, created-at, and optionally the raw serialized value
• SolidStackWeb.allow_value_preview config toggle (default false) gates the value section; when enabled, displays up to 4 KB with JSON pretty-printing and a format badge (JSON / Text)
• Entry key in the browser table is now a link to the detail page
• Delete button on the detail page redirects back to the entry list
• allow_value_preview = true set in the dummy app initializer for local testing

Test plan

• bundle exec rake passes (219 examples, 0 failures)
• Browse to a cache entry detail page with allow_value_preview = false — value section shows the disabled message with config hint
• Set allow_value_preview = true in dummy app — JSON values render pretty-printed with JSON badge; plain-text values render with Text badge
• Delete button on detail page removes the entry and redirects to /cache/entries
• Key link in the entries table navigates to the detail page

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.00%. Comparing base (0840f2b) to head (057a708).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
app/helpers/solid_stack_web/application_helper.rb	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
- Coverage   99.19%   99.00%   -0.19%     
==========================================
  Files          30       30              
  Lines         496      505       +9     
==========================================
+ Hits          492      500       +8     
- Misses          4        5       +1     

Files with missing lines	Coverage Δ
...ollers/solid_stack_web/cache_entries_controller.rb	100.00% <100.00%> (ø)
lib/solid_stack_web.rb	100.00% <100.00%> (ø)
app/helpers/solid_stack_web/application_helper.rb	97.87% <80.00%> (-2.13%)	⬇️

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