feat(auth): add a narrow ingest perimeter scope - #363

Problem

D62 needs a browser to add one document without gaining either read access to memory or authority to create a standing connector. The perimeter previously had only read and write, so POST /ingest required the broad write scope.

What this does

• adds the closed ingest perimeter scope and maps only POST /ingest to it
• keeps write as the full scope that also satisfies ingest; keeps read and ingest disjoint
• exercises real signed credentials at the HTTP perimeter, including connector/search/operation refusals and the write-token compatibility path
• preserves fail-closed WRITE classification for unenumerated routes

How verified

• uv run pytest src/tests/surfaces/test_route_scope.py src/tests/adapters/test_signed_token_auth.py src/tests/surfaces/test_openapi_export.py -q — 73 passed after rebase
• uv run lint-imports
• uv run ruff check src/ benchmarks/
• uv run ruff format --check src/ benchmarks/
• uv run pyright src/ benchmarks/
• uv run python scripts/export_openapi.py -o openapi.json — 25 routes; no schema diff
• full uv run pytest src/tests -q — 2,198 passed, 2 skipped, 1 failed locally after 1h58m; the failure is the pre-existing/out-of-scope macOS MIME assertion test_sdk_pushes_lineage_metadata_to_e0 (.md detected as application/octet-stream). A focused rerun reproduced it. Per the package safety rule, this PR does not touch MIME routing or PR feat(e0): refuse an unroutable MIME at the E0 gate instead of dead-lettering it (D104) #342; Linux CI is the authoritative full-suite result.

Docs

website/src/app/docs/configuration/page.mdx now documents all three signed-credential scopes and their exact reach.

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