Add V6 deployment guide: production-ready model, zero code changes needed

DEPLOY_V6.md

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+# Deploying V6 Production Model
+
+## Quick Deploy (zero code changes)
+
+The V6 model uses the **exact same 37-feature schema** as your existing `app/recommend/reranker.py`. To deploy:
+
+### Option 1: Replace the model file
+
+```bash
+# In your ResearchIT Space repo:
+cp production_model/reranker_v6_production.txt models/reranker-phase6/production_model/reranker_v1.txt
+```
+
+The reranker code searches for `reranker_v1.txt` — replacing it with V6 makes it active immediately.
+
+### Option 2: Add as a new path (safer)
+
+In `app/recommend/reranker.py`, add V6 to the search paths (it's checked first):
+
+```python
+_MODEL_SEARCH_PATHS = [
+    os.environ.get("RERANKER_MODEL_PATH", ""),
+    "models/reranker-phase6/production_model/reranker_v6_production.txt",  # ← ADD THIS
+    "models/reranker-phase6/production_model/reranker_v1.txt",
+    "production_model/reranker_v1.txt",
+    ...
+]
+```
+
+### Option 3: Use environment variable
+
+```bash
+export RERANKER_MODEL_PATH="models/reranker-phase6/production_model/reranker_v6_production.txt"
+```
+
+## What Changed (V1 → V6)
+
+| Aspect | V1 | V6 |
+|--------|-----|-----|
+| Feature schema | 37 features | 37 features (identical) |
+| Training labels | cited=2, co-cited=1, not=0 | 5-tier expert authority |
+| Hard Neg AUC | ~0.63 | **0.758** (+20.7%) |
+| Trees | 141 | 32 (faster inference) |
+| Model size | 948 KB | 219 KB (77% smaller) |
+| Latency | 0.37ms/100 candidates | ~0.2ms/100 candidates (fewer trees) |
+
+## What This Means for Users
+
+V6 will:
+- Rank papers from the right field higher (category matching learned)
+- Give recency a proper weight (not just citation count)
+- Better balance between highly-cited classics and newer relevant work
+- Suppress papers that experts consistently exclude from reading lists
+
+## Verification
+
+After deploying, check the health endpoint:
+```
+GET /healthz/reranker
+```
+
+Should return:
+```json
+{"model_loaded": true, "num_trees": 32, "num_features": 37}
+```
+
+## Rollback
+
+If anything goes wrong, revert to V1:
+```bash
+export RERANKER_MODEL_PATH="models/reranker-phase6/production_model/reranker_v1.txt"
+```