[Jemade]

Actual changes

• Replace non-atomic read/check/add Redis sliding-window sequence with WATCH/MULTI optimistic transactions, retrying conflicting updates.
• Avoid loading every window entry into memory; count active entries and retrieve only oldest score.
• Add concurrent quota regression test and configuration validation tests.
• Retry webhook HTTP 429/5xx and transport failures, not permanent HTTP 4xx errors; add regression test.

Validation status

Changes committed but not test-executed. Local GitHub clone failed because DNS resolution of github.com is unavailable in the execution container. Run pip install -e '.[dev]', ruff check ., and pytest -q, including tests with the project's fakeredis version. Do not merge before green CI.

Still required for high-scale public deployment

• Strong SSRF protection for user-supplied webhook URLs (DNS resolution and rebinding, private-network blocking, redirects, and proxy behavior).
• Durable outbox/background webhook workers; currently request path waits on delivery.
• Shared HTTP client lifecycles, bounded provider concurrency, idempotency keys, per-tenant quota/budget enforcement, and load tests.
• Reconsider production Redis fail-open policy, security controls and database pool sizing.

This PR is a targeted reliability improvement, not a claim of production scalability.