docs: add ML service usage guide

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

apps/ml-service/README.md

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+# ZeaVis ML Service
+
+FastAPI service for serving corn leaf disease predictions from the trained Keras model.
+
+## Setup
+
+```bash
+python -m venv .venv
+source .venv/bin/activate
+pip install -r requirements.txt
+```
+
+The default model path is `../../Machine_Learning/best_model/best_model.keras`. Override it with `MODEL_PATH` if needed.
+
+## Run
+
+```bash
+uvicorn main:app --host 0.0.0.0 --port 8001
+```
+
+## Endpoints
+
+- `GET /health` — service status and model loaded status.
+- `GET /metadata` — service name, version, labels, input size, model path, and model loaded status.
+- `POST /predict` — multipart image upload for disease classification.
+
+## Verify
+
+```bash
+curl http://localhost:8001/health
+curl http://localhost:8001/metadata
+curl -X POST http://localhost:8001/predict -F "file=@/path/to/corn-leaf.jpg"
+```