Add inventory relations model and API structure

backend/app/routers/relations.py

@@ -0,0 +1,37 @@
+from fastapi import APIRouter, HTTPException, Query
+
+from app.schemas import EntityResponse, ListResponse, Relation
+from app.services.inventory import get_node_relations
+from app.services.store import RELATIONS
+
+router = APIRouter(prefix='/relations', tags=['relations'])
+
+
+@router.get('', response_model=ListResponse[Relation])
+def list_relations(
+    source_id: str | None = None,
+    target_id: str | None = None,
+    layer: str | None = None,
+    type: str | None = Query(default=None),
+    min_confidence: float | None = None,
+) -> ListResponse[Relation]:
+    items = RELATIONS
+    if source_id:
+        items = [r for r in items if r.source_id == source_id]
+    if target_id:
+        items = [r for r in items if r.target_id == target_id]
+    if layer:
+        items = [r for r in items if r.layer == layer]
+    if type:
+        items = [r for r in items if r.type == type]
+    if min_confidence is not None:
+        items = [r for r in items if r.confidence >= min_confidence]
+    return ListResponse(count=len(items), items=items)
+
+
+@router.get('/{relation_id}', response_model=EntityResponse[Relation])
+def get_relation(relation_id: str) -> EntityResponse[Relation]:
+    for item in RELATIONS:
+        if item.id == relation_id:
+            return EntityResponse(item=item)
+    raise HTTPException(status_code=404, detail='Relation not found')