Fix metric imports: use module-level access instead of name imports

Type-annotated metric variables (e.g. `geojson_cache_operations: Counter`)
don't exist as importable names until init_metrics() runs.  Switch all
`from api.metrics import <metric>` to `import api.metrics as m` and
access instruments as attributes at runtime to avoid ImportError.

listing_processor.py

@@ -326,10 +326,10 @@ class DetectFloorplanStep(Step):
 
         # Record OCR metrics
         try:
-            from api.metrics import ocr_attempts, ocr_successes
-            ocr_attempts.add(1)
+            import api.metrics as m
+            m.ocr_attempts.add(1)
             if max_sqm > 0:
-                ocr_successes.add(1)
+                m.ocr_successes.add(1)
         except Exception:
             pass  # Metrics not initialised