chore: update .gitignore to exclude sensitive files

- Add *.db-shm and *.db-wal patterns for SQLite journal files
- Add perf-test-*.properties pattern for credential files

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: sionsmith

.gitignore

@@ -6,6 +6,11 @@ Cargo.lock
 # Backup data
 /backups/
 *.db
+*.db-shm
+*.db-wal
+
+# Performance test properties with credentials
+perf-test-*.properties
 
 # Config files with credentials (keep examples, ignore real configs)
 config/confluent-*.yaml