feat: add anonymous access configuration for KV endpoints (issue #5)

- Add AllowAnonymousRead and AllowAnonymousWrite config parameters
- Set both to false by default for security
- Apply conditional authentication middleware to KV endpoints:
  - GET requires auth if AllowAnonymousRead is false
  - PUT requires auth if AllowAnonymousWrite is false
  - DELETE always requires authentication (no anonymous delete)
- Update integration tests to enable anonymous access for testing
- Maintain backward compatibility when AuthEnabled is false

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

config/config.go

@@ -55,6 +55,10 @@ func Default() *types.Config {
 		ClusteringEnabled:      true,
 		RateLimitingEnabled:    true,
 		RevisionHistoryEnabled: true,
+		
+		// Default anonymous access settings (both disabled by default for security)
+		AllowAnonymousRead:     false,
+		AllowAnonymousWrite:    false,
 	}
 }