fix(demo): deduplicate materialization and parallelize result uploads

[suguanYang]

Summary

• Add short-lived demo materialization claims with stable 409 responses for completed/in-progress duplicates.
• Narrow namespace snapshot locking so different demo sources can publish concurrently.
• Batch publication writes and reuse a process-level S3 client/upload executor.
• Upload raw result files with bounded process-level concurrency (default 20) and increase the shared S3 pool to 20.

Verification

• Demo contract tests: 10 passed
• Result storage focused tests: 4 passed
• Storage config contract: 7 passed
• Ruff: passed
• Pyright: 0 errors
• git diff --check: passed
• Local TSLA bundle timing: median 10.25s for 73 files / ~92 MiB on LocalStack

Notes

• Full worker contract suite requires local pytest_postgresql/fakeredis dependencies and was not rerun in this environment.
• Production S3 throughput still needs post-deploy internal-network measurement.