Fill Python reliability pages and api-reference/worker #530

Python docs missing vs node/java:

• api-reference/worker
• guides/reliability/: concurrency, dead-letter, timeouts (py has only index + rate-limiting; node/java each have 5 pages)
• guides/core/: cancellation, enqueue-options

Found in the 2026-07-24 SDK parity audit (SDK_PARITY_REPORT.md).