Add phase 2 of the insight chart color registry (slug threading + resolver)

[yellowcap]

Summary

Phase 2 of the insight chart color plan (see docs/insight-chart-colors-plan.md, follows phase 1 in #771). Today the code-executor LLM writes already-translated category labels straight into chart_data, with no stable key surviving translation. This threads a stable category slug through the pipeline and resolves colors deterministically (never LLM-generated) against the phase-1 registry, so chart colors stay consistent across languages and match the map legend.

Changes

• EXECUTOR_WORKFLOW (src/agent/subagents/analyst/prompts.py): instructs the code executor to emit a {column}__slug sibling column next to any categorical color column, using canonical slugs from the dataset's registry when available.
• Analyst._resolve_charts / build_analysis_prompt (src/agent/subagents/analyst/tool.py): passes category_slug_hints (from get_dataset_palette(dataset_id)) into the prompt and threads dataset_id through.
• New resolve_chart_colors() (src/agent/subagents/analyst/charts/color_resolver.py): deterministic, hash-based fallback color for slugs with no registry entry; attaches color_map/series_color/divergent_colors onto InsightChart.
• update_insight_display (src/agent/tools/update_insight_display.py): re-runs the resolver on revision, using the dataset_id persisted on the original chart (revisions can re-map color_field to a different column).
• New DB columns on insight_charts via migration d4f7b1e9a3c2_add_chart_color_registry_fields: dataset_id, color_map, series_color, divergent_colors.
• InsightChart model/schema/API surface carries the new fields through both acquisition paths (chat stream + analysis LRO).

Compatibility

Additive: new nullable/defaulted DB columns, new optional model fields, new prompt section. Existing charts without a dataset_id resolve to empty/None colors (falls back to current frontend behavior) until the companion project-zeno-next PR consumes colorMap/seriesColor/divergentColors.

Test plan

• uv run pytest tests/agent/test_update_insight_display.py tests/tools/test_generate_insights_tiered.py tests/unit/agent/tools/test_color_resolver.py tests/unit/agent/tools/test_insight_chart_model.py tests/api/test_insights.py
• ruff / ruff-format / mypy clean on touched files

[srmsoumya]

Conceptually this looks good to me @yellowcap - colors are resolved deterministically and persisted with dataset_id for re-resolution on revision.

One thing to check: stack_field / group_field don't drive color resolution. _categorical_column only looks at color_field (and x_axis for pies), and the prompt's slug instruction also only mentions color_field. So a stacked-bar chart with x=year, stack=driver and no color_field might not get a color_map.