feat(retrieval): add voyage rerank-2 cross-encoder stage (feature flag)

Stage B of voyage-upgrades-plan rewritten: instead of context-3 (which
4 POCs showed inconsistent improvement), add a cross-encoder rerank
layer on top of voyage-3. Default off (VOYAGE_RERANK_ENABLED=false).

POC validation (785-doc corpus, 12 queries, claude-haiku-4-5 judge):
- mean@3 +4.5% (4.306 → 4.500)
- practical-category queries +11.6% (3.78 → 4.22)
- latency +702ms per query
- no schema change, no re-embed, no double storage

Plumbing:
- config: VOYAGE_RERANK_ENABLED / _MODEL / _FETCH_K env vars
- embeddings.voyage_rerank() wraps voyageai client.rerank
- services/rerank.py: maybe_rerank() helper — fetches FETCH_K candidates
  via the bi-encoder then reranks to top-K. Fail-open if Voyage rerank is
  unavailable.
- tools/search.py: search_decisions, search_case_documents,
  find_similar_cases all wrapped
- services/precedent_library.search_library wrapped

Smoke-tested locally with flag on/off — produces expected behaviour and
latency profile. Ready for production rollout via Coolify env flip after
deploy.

POCs (kept under scripts/ for reference):
- voyage_context3_poc{_long}.py — context-3 evaluation (rejected)
- voyage_multimodal_poc.py — multimodal-3 (stage C, deferred)
- voyage_rerank_judge_poc.py — single-case rerank benchmark
- voyage_rerank_corpus_poc.py — full-corpus rerank validation

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

mcp-server/src/legal_mcp/config.py

@@ -47,6 +47,17 @@ VOYAGE_API_KEY = os.environ.get("VOYAGE_API_KEY", "")
 VOYAGE_MODEL = os.environ.get("VOYAGE_MODEL", "voyage-law-2")
 VOYAGE_DIMENSIONS = 1024
 
+# Rerank — cross-encoder second-stage. Off by default; flip with env to
+# enable across all semantic search tools (search_decisions,
+# search_case_documents, find_similar_cases, search_precedent_library).
+VOYAGE_RERANK_MODEL = os.environ.get("VOYAGE_RERANK_MODEL", "rerank-2")
+VOYAGE_RERANK_ENABLED = (
+    os.environ.get("VOYAGE_RERANK_ENABLED", "false").lower() == "true"
+)
+# How many candidates to fetch from bi-encoder before reranking.
+# 50 was the depth used in the POC; balances recall vs rerank cost.
+VOYAGE_RERANK_FETCH_K = int(os.environ.get("VOYAGE_RERANK_FETCH_K", "50"))
+
 # Google Cloud Vision (OCR for scanned PDFs)
 GOOGLE_CLOUD_VISION_API_KEY = os.environ.get("GOOGLE_CLOUD_VISION_API_KEY", "")