fix(api): accept verbatim extraction mode in bank manifests

[ishanmalik]

Problem

The engine accepts retain_extraction_mode=verbatim through environment/config resolution, but bank-template manifest import rejected the same mode because the validator had a re-typed mode tuple that omitted it. This surfaced during a verbatim-mode data migration.

Solution

Use the canonical RETAIN_EXTRACTION_MODES tuple for bank-template validation so future extraction modes cannot drift, and update the related API schema descriptions to include verbatim and chunks.

Testing

• uv run --directory hindsight-api-slim pytest tests/test_bank_templates.py::TestImportValidation::test_verbatim_extraction_mode_is_valid
• uv run --directory hindsight-api-slim python -m py_compile hindsight_api/api/http.py
• scripts/generate-bank-template-schema.sh
• scripts/generate-docs-skill.sh

[ishanmalik]

Follow-up: re-ran scripts/generate-openapi.sh with full docs dependencies installed — build validates and the regenerated artifacts match the committed ones exactly (no diff). The client-description strings updated manually in this PR are likewise identical to generator output.

[benfrank241]

thanks @ishanmalik good catch.