Hybrid search across manual pages

Search manual pages using hybrid ranking (Postgres FTS + pgvector semantic similarity merged via Reciprocal Rank Fusion). Falls back to FTS-only if embeddings are unavailable. Supports case-insensitive substring filtering by model_number (searched in applicable_models), equipment_type, manufacturer, document_id, equipment_family, and document_type. Each result includes a snippet and nested document and equipment metadata. Results include normalized_rank (score relative to top result) alongside the raw rank. low_confidence is true when the best vector cosine similarity is below 0.10 (applies in both vector and hybrid modes). Default limit: 5 results. Pass debug=true to include rel_pos (relative page position 0.0–1.0) on each result for inspecting positional ranking behavior.