feat(cases): enforce purpose-bound case governance

src/govoplan_cases/backend/evidence_links.py

@@ -9,7 +9,7 @@ from urllib.parse import parse_qs, quote, urlsplit
 from sqlalchemy.orm import Session
 
 from govoplan_core.core.institutional import EvidenceReference
-from govoplan_cases.backend.domain import CaseRecord
+from govoplan_cases.backend.domain import DEFAULT_CASE_ACCESS_PURPOSE, CaseRecord
 from govoplan_cases.backend.service import CaseStoreError, get_case, update_case
 
 
@@ -31,12 +31,14 @@ def link_case_evidence(
     recorded_at: datetime,
     change_reason: str,
     idempotency_key: str,
+    purpose: str = DEFAULT_CASE_ACCESS_PURPOSE,
 ) -> CaseRecord:
     base = _base_revision(
         session,
         principal,
         case_id=case_id,
         expected_revision=expected_revision,
+        purpose=purpose,
     )
     evidence = _validated_evidence(reference, tenant_id=base.reference.tenant_id)
     if any(item.evidence_id == evidence.evidence_id for item in base.evidence_refs):
@@ -50,6 +52,7 @@ def link_case_evidence(
         recorded_at=recorded_at,
         change_reason=change_reason,
         idempotency_key=idempotency_key,
+        purpose=purpose,
     )
 
 
@@ -63,12 +66,14 @@ def unlink_case_evidence(
     recorded_at: datetime,
     change_reason: str,
     idempotency_key: str,
+    purpose: str = DEFAULT_CASE_ACCESS_PURPOSE,
 ) -> CaseRecord:
     base = _base_revision(
         session,
         principal,
         case_id=case_id,
         expected_revision=expected_revision,
+        purpose=purpose,
     )
     evidence = next(
         (item for item in base.evidence_refs if item.evidence_id == evidence_id),
@@ -89,6 +94,7 @@ def unlink_case_evidence(
         recorded_at=recorded_at,
         change_reason=change_reason,
         idempotency_key=idempotency_key,
+        purpose=purpose,
     )
 
 
@@ -119,12 +125,14 @@ def _base_revision(
     *,
     case_id: str,
     expected_revision: int,
+    purpose: str,
 ) -> CaseRecord:
     base = get_case(
         session,
         principal,
         case_id=case_id,
         revision=expected_revision,
+        purpose=purpose,
     )
     if base is None:
         raise LookupError("Case revision not found.")