prep docs and service updates

docs/modules/excititor/prep/2025-11-20-graph-21-005-prep.md

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+# Excititor · Graph Indexes Prep (GRAPH-21-005)
+
+- **Date:** 2025-11-20
+- **Depends on:** GRAPH-21-002 overlays
+- **Working directory:** `src/Excititor/__Libraries/StellaOps.Excititor.Storage.Mongo`
+
+## Index plan
+- Collection: `vex_observations`
+  - Compound index `{ tenant: 1, component.purl: 1, advisoryId: 1, source: 1, modifiedAt: -1 }` (supports overlay queries and truncation cursor).
+  - Sparse index `{ tenant: 1, component.purl: 1, status: 1 }` for summary counts.
+- Collection: `vex_overlays` (materialized cache, optional)
+  - Index `{ tenant: 1, purl: 1 }` unique.
+  - TTL index on `cachedAt` configurable via `excititor:graph:overlayTtlSeconds`.
+
+## Determinism
+- Materialization job must sort observations as per GRAPH-21-001 ordering before writing overlays so pagination/cursors align.
+- TTL applied identically across tenants; default 300s, override allowed via config but must be documented.
+
+## Acceptance for prep completion
+- Index keys and TTL knobs defined; downstream storage tasks can implement without further contract churn.
+