fix: re-enable async logging with proper shutdown flushing

[ivanauth]

Fixes #889

Async logging was disabled because nothing was flushing the diode.Writer buffer on shutdown - logs were getting lost. The cobrazerolog library hides the closer so we couldn't fix it there.

This creates our own logging setup that exposes the closer, stores it globally, and flushes it via defer log.Close() in main. Also restructured main() to return an int so defers actually run before exit.

[codecov]

Codecov Report

❌ Patch coverage is 85.61151% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.64%. Comparing base (fa85ab8) to head (96b842e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/spicedb/main.go	0.00%	11 Missing ⚠️
pkg/cmd/server/logging_run_e.go	88.24%	4 Missing and 2 partials ⚠️
internal/logging/logger.go	95.84%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2779      +/-   ##
==========================================
- Coverage   73.66%   73.64%   -0.01%     
==========================================
  Files         499      500       +1     
  Lines       60165    60307     +142     
==========================================
+ Hits        44312    44409      +97     
- Misses      12666    12700      +34     
- Partials     3187     3198      +11     

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[tstirrat15]

We own cobrautil - it might make sense to fix it there.

[ivanauth]

Update: cobrautil fix submitted

Instead of duplicating cobrautil's logging setup, I've submitted a fix upstream: jzelinskie/cobrautil#66

That PR:

• Exposes Builder.Close() to flush the async diode writer on shutdown
• Fixes WithAsync(size, pollInterval) params being ignored (hardcoded to 1000/10ms)
• Wraps os.Stderr to prevent diode.Writer.Close() from closing the fd
• Guards against double RunE() leaking poll goroutines
• Defaults size to 1000 if <= 0 to prevent panic

Once that's merged and tagged, this PR can be significantly simplified:

• Delete logging_run_e.go and logging_run_e_test.go (no need to duplicate cobrautil's setup)
• Delete noCloseWriter (cobrautil now handles this)
• Simplify internal/logging/logger.go — store the *Builder ref, call builder.Close()
• Keep the main() → run() int restructuring (so defers run before os.Exit)
• Bump cobrautil dependency

cc @tstirrat15 per your suggestion to fix it in cobrautil