Add batch agent review flow

[cgasgarth]

Summary

• add a lighttable-launched batch review flow for 1-10 selected images that reuses the existing darkroom agent pipeline and tags queued, edited, and failed images for review
• add backend batch orchestration and /v1/chat/batch support with review metadata, plus guardrails that keep batch requests single-turn and reuse the shared planner bridge
• cover the new backend behavior with focused protocol, API, and orchestrator tests, and fix unhappy-path batch cleanup found during review

Validation

• uv run --extra dev pytest server/tests
• uv run --extra dev pyright server shared
• npm run darktable:build
• darktable/build-5.4.1/bin/tests/darktable-test-agent-catalog
• darktable/build-5.4.1/bin/tests/darktable-test-variables (fails in this environment with can't init develop system, aborting)