chore(docker)(deps): bump python from 3.9-slim-buster to 3.11-slim-buster

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Bumps python from 3.9-slim-buster to 3.11-slim-buster.

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[ringo380]

Hold for individual review — this bump's scope is wider than the title suggests. It also bumps Dockerfile.web and Dockerfile.worker from Python 3.11 → 3.14 (released Oct 2025). For Dockerfile.worker specifically that's risky: TensorFlow officially supports up through 3.13, PyTorch through 3.13. 3.14 may work but isn't tested by either upstream. Suggest splitting into two PRs: (1) Dockerfile only (3.9 → 3.11, safe), and (2) Dockerfile.web/.worker → 3.12 or 3.13 (one cadence below latest, ecosystem-safe). After that the 3.14 bump can be picked up once the heavy ML stack officially supports it.