P6.1 follow-on: unbox BuildingPartIdentifier at backend boundaries

Threads the strict BuildingPartIdentifier type (introduced in a8b443f6)
through the two remaining backend touchpoints:

- EpcBuildingPartModel.from_*: SQLModel column expects a string, so
  unbox the enum with .identifier.value before binding to the DB.
- documents_parser end-to-end tests: swap bare-string equality
  ("main" / "extension_1") for identity checks against the enum
  members (BuildingPartIdentifier.MAIN / EXTENSION_1).

Documents_parser test pack passes (105/105). No dedicated SQLModel test
covers EpcBuildingPartModel.from_*; the .value line is exercised
transitively via db_writer.py / local_runner.py in production.

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

backend/app/db/models/epc_property.py

@@ -501,7 +501,7 @@ class EpcBuildingPartModel(SQLModel, table=True):
         aw2 = part.sap_alternative_wall_2
         return cls(
             epc_property_id=epc_property_id,
-            identifier=part.identifier,
+            identifier=part.identifier.value,
             construction_age_band=part.construction_age_band,
             wall_construction=str(part.wall_construction),
             wall_insulation_type=str(part.wall_insulation_type),

backend/documents_parser/tests/test_elmhurst_end_to_end.py

@@ -5,7 +5,7 @@ from datetime import date
 import pytest
 
 from backend.documents_parser.elmhurst_extractor import ElmhurstSiteNotesExtractor
-from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from datatypes.epc.domain.epc_property_data import BuildingPartIdentifier, EpcPropertyData
 from datatypes.epc.domain.mapper import EpcPropertyDataMapper
 
 FIXTURE_PATH = os.path.join(
@@ -130,7 +130,7 @@ class TestBuildingPart:
         assert len(result.sap_building_parts) == 1
 
     def test_identifier(self, result: EpcPropertyData) -> None:
-        assert result.sap_building_parts[0].identifier == "main"
+        assert result.sap_building_parts[0].identifier is BuildingPartIdentifier.MAIN
 
     def test_construction_age_band(self, result: EpcPropertyData) -> None:
         assert result.sap_building_parts[0].construction_age_band == "1950-1966"

backend/documents_parser/tests/test_end_to_end.py

@@ -6,6 +6,7 @@ import pytest
 from backend.documents_parser.extractor import PasHubRdSapSiteNotesExtractor
 from backend.documents_parser.pdf import pdf_to_text_list
 from datatypes.epc.domain.epc_property_data import (
+    BuildingPartIdentifier,
     EpcPropertyData,
     InstantaneousWwhrs,
     MainHeatingDetail,
@@ -187,7 +188,7 @@ class TestPdfToEpcPropertyData:
             ),
             sap_building_parts=[
                 SapBuildingPart(
-                    identifier="main",
+                    identifier=BuildingPartIdentifier.MAIN,
                     construction_age_band="1950-1966",
                     wall_construction="Cavity",
                     wall_insulation_type="Filled Cavity",
@@ -218,7 +219,7 @@ class TestPdfToEpcPropertyData:
                     floor_u_value_known=False,
                 ),
                 SapBuildingPart(
-                    identifier="extension_1",
+                    identifier=BuildingPartIdentifier.EXTENSION_1,
                     construction_age_band="2003-2006",
                     wall_construction="Cavity",
                     wall_insulation_type="As built",