fix(recall): fall back when optional reranker fails

[oldnicke]

Summary

• add HINDSIGHT_API_RERANKER_REQUIRED as a static server setting that defaults to true
• when explicitly disabled, fall back to RRF-based scoring after reranker initialization or prediction failures
• surface degraded recalls in search traces, OpenTelemetry attributes, warning logs, and null reranker scores
• preserve cancellation propagation and skip min_scores.reranker only when no real reranker score exists

Compatibility

The default remains strict, so existing deployments continue to fail recall when the reranker fails. Fail-open behavior requires HINDSIGHT_API_RERANKER_REQUIRED=false.

Tests

• 79 related reranker, scoring, trace, timeout, and min-score tests
• ./scripts/hooks/lint.sh
• ./scripts/hooks/check-unused.sh
• uv run ty check hindsight_api/config.py hindsight_api/engine/memory_engine.py
• ./scripts/hooks/generate-docs-skill.sh

Generated files

Generated documentation references were refreshed from the latest main sources, so this PR can pass generated-file verification independently.

Fixes #3168

[nicoloboschi]

superseded by #3176