feat(external): add Atlas Cloud image provider

[binyangzhu000-sudo]

Summary

Adds Atlas Cloud as a first-class external image generation provider.

• Implements the Atlas Cloud asynchronous submit, poll, and image download flow with rate-limit and response validation.
• Adds API key/base URL configuration, dependency registration, and provider availability reporting.
• Adds an Atlas Cloud image generation invocation and a FLUX.1 Schnell starter model.
• Regenerates the frontend API schema and adds focused provider contract tests.

Related Issues / Discussions

None.

QA Instructions

• uv tool run ruff@0.11.2 format --check <changed Python files>
• uv tool run ruff@0.11.2 check <changed Python files>
• python -m compileall <changed Python files>
• pytest tests/app/services/external_generation/test_atlascloud_provider.py tests/app/services/external_generation/test_seedream_provider.py tests/app/services/external_generation/test_alibabacloud_provider.py tests/app/services/external_generation/test_external_provider_adapters.py -q (38 passed)
• python scripts/generate_openapi_schema.py | pnpm typegen (deterministic on a second run)
• pnpm lint:tsc
• pnpm exec prettier --check src/services/api/schema.ts
• python -m pip wheel . --no-deps (wheel built successfully)

Merge Plan

No special merge steps required.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)