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 85.08772% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.72%. Comparing base (7519ee2) to head (3b4d309).

Files with missing lines	Patch %	Lines
cmd/spicedb/main.go	0.00%	10 Missing ⚠️
pkg/cmd/server/logging_run_e.go	88.89%	3 Missing and 2 partials ⚠️
internal/logging/logger.go	96.56%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2779      +/-   ##
==========================================
+ Coverage   77.72%   77.72%   +0.01%     
==========================================
  Files         472      473       +1     
  Lines       49716    49831     +115     
==========================================
+ Hits        38636    38728      +92     
- Misses       8228     8247      +19     
- Partials     2852     2856       +4     

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