Add reranking, RRF fusion, bench harness, tag contexts, and data ingestion

Implements five of the six enhancements from docs/kb-enhancements-proposal.htm,
closing the retrieval-quality gap identified in the qmd review.

- Cross-encoder reranking: new kb/reranker.py loads an optional reranking
  model at startup (KB_RERANK_ENABLED, KB_RERANKER_MODEL,
  KB_RERANK_CANDIDATES). Search degrades gracefully to plain hybrid
  retrieval when the model is absent. Exposed via a "rerank" block in
  /status, a rerank flag on search, and --no-rerank in the CLI.
- RRF rank fusion: FTS and vector lists now merge by reciprocal rank
  fusion with a top-rank bonus, replacing the old score blend. Scores are
  comparable across queries.
- Bench harness and explain traces: kb bench runs a query fixture against
  each backend and reports precision@k, recall and MRR. --explain returns a
  per-result score breakdown.
- Tag context descriptions: tags carry an optional one-line description
  (kb tag-describe), returned as tag_contexts with search results. Adds a
  tags.description column migration.
- Structured data ingestion: .json/.yaml/.toml files ingest as text via the
  new "data" doc type, pretty-printing minified JSON before chunking.

Query expansion (proposal item 5) is deliberately left out pending bench
results. Requires engine v3.3.0.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

engine/main.py

@@ -40,6 +40,18 @@ async def lifespan(app: FastAPI):
     init_schema(conn, model_dim)
     conn.close()
 
+    # Optional reranker — search degrades gracefully if this fails
+    if cfg.rerank_enabled:
+        from kb.reranker import load_reranker
+        try:
+            load_reranker(cfg.reranker_model, cfg.device)
+        except Exception:
+            log.warning(
+                "Failed to load reranker '%s' — searches will not be reranked",
+                cfg.reranker_model,
+                exc_info=True,
+            )
+
     # Start background ingestion worker
     worker_task = asyncio.create_task(ingestion_worker())