fix(ocr): narrow exception handling and add unit tests for ensure_blla_model

- _model_is_loadable: narrow bare except to (RuntimeError, OSError, ValueError)
  with DEBUG-level fallback for unexpected exceptions — prevents silent masking
  of missing kraken install or AttributeError on vgsl
- _run_segtrain: replace bare except:pass with log.warning so height-check
  fallback is visible in container logs
- New test_ensure_blla_model.py: covers model-OK early return, incompatible
  model rename+replace, and missing model download paths

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

ocr-service/main.py

@@ -494,8 +494,8 @@ async def segtrain_model(
                             "will train from scratch at 800px",
                             _m.input[2],
                         )
-                except Exception:
-                    pass
+                except Exception as exc:
+                    log.warning("Could not inspect base model height, training from scratch: %s", exc)
 
             if use_base_model:
                 cmd += ["-i", blla_model_path, "--resize", "union", "-s", seg_spec]