Enhance risk API documentation and error handling

- Updated API documentation for risk endpoints to include optional caching headers and error catalog references.
- Added a new error catalog JSON file to standardize error responses.
- Improved explainability documentation with sample outputs for console and CLI.
- Added SHA256 checksums for new sample files related to explainability.
- Refined AocGuard tests to utilize a helper method for generating test JSON, improving readability and maintainability.
- Updated runbook references to ensure consistency in sprint documentation.
- Introduced stub implementations for MongoDB storage interfaces and options, laying groundwork for future development.
- Disabled analytics in Angular CLI configuration for privacy considerations.

docs/risk/explainability.md

@@ -16,8 +16,8 @@
 - UI/CLI expectations: deterministic ordering (factor type → source → timestamp), highlight top contributors, show attestation status for each factor.
 
 ## UI/CLI Views
-- Console: table of factors sorted by contribution, severity badge, gate badges (e.g., KEV+reachability), link to provenance hashes.
-- CLI `stella risk explain job-001`: render table using fixture `explain-trace.json`; include `--json` option that emits the same payload.
+- Console: frame sample in `docs/risk/samples/explain/console-frame.json` shows top contributors, gate badges, and provenance hashes.
+- CLI `stella risk explain job-001`: deterministic text fixture in `docs/risk/samples/explain/cli-explain.txt`; `--json` mirrors `explain-trace.json`.
 - Export Center: embed explain payload + SHA256 manifest; CSV export keeps deterministic ordering.
 
 ## Determinism & Offline Posture