feat(ocr): flip ocr_models_ready to 1 once the lifespan startup finishes

Mirrors the existing _models_ready bool so Prometheus has a time-series
liveness/readiness signal for future alerting rules (e.g.
ocr_models_ready < 1 for 2m).

Refs #652 (AC7)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ocr-service/main.py

@@ -69,6 +69,7 @@ async def lifespan(app: FastAPI):
     kraken_engine.load_models()
     load_spell_checker()
     _models_ready = True
+    metrics.ocr_models_ready.set(1)
     logger.info("Startup complete — ready to accept requests")
 
     yield

ocr-service/test_metrics.py

@@ -422,3 +422,23 @@ async def test_ocr_model_accuracy_gauge_set_per_kind_after_successful_training(f
 
     assert fresh_metrics.ocr_model_accuracy.labels(kind="recognition")._value.get() == pytest.approx(recognition_accuracy)
     assert fresh_metrics.ocr_model_accuracy.labels(kind="segmentation")._value.get() == pytest.approx(segmentation_accuracy)
+
+
+def test_ocr_models_ready_gauge_defaults_to_zero():
+    """A freshly-built OcrMetrics has ocr_models_ready=0 before lifespan runs."""
+    metrics = build_metrics(CollectorRegistry())
+    assert metrics.ocr_models_ready._value.get() == 0.0
+
+
+@pytest.mark.asyncio
+async def test_ocr_models_ready_gauge_is_one_after_lifespan_startup(fresh_metrics):
+    """The lifespan flips ocr_models_ready to 1 once load_models / load_spell_checker return.
+
+    ASGITransport does not run lifespan by default, so the lifespan context
+    manager is driven directly to exercise the startup code path.
+    """
+    assert fresh_metrics.ocr_models_ready._value.get() == 0.0
+    with patch("main.kraken_engine.load_models"), \
+         patch("main.load_spell_checker"):
+        async with app.router.lifespan_context(app):
+            assert fresh_metrics.ocr_models_ready._value.get() == 1.0