feat: add reusable solution library

backend/apps/solutions/tests/__init__.py

@@ -0,0 +1 @@
+

backend/apps/solutions/tests/test_solution_api.py

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+from apps.tickets.models import Ticket
+
+
+def test_create_and_list_solution(client, db):
+    response = client.post(
+        "/api/solutions",
+        data={
+            "title": "Restart callback worker",
+            "problem_signature": "HTTP 500 from callback",
+            "root_cause": "Worker connection pool stale",
+            "solution": "Restart callback worker and verify queue depth.",
+            "status": "published",
+        },
+        content_type="application/json",
+    )
+
+    assert response.status_code == 200
+    solution_id = response.json()["id"]
+
+    response = client.get("/api/solutions")
+    assert response.status_code == 200
+    assert response.json()[0]["id"] == solution_id
+
+
+def test_link_solution_to_ticket_increments_usage(client, db):
+    ticket = Ticket.objects.create(ticket_no="TK202606300020", title="A", description="A")
+    solution = client.post(
+        "/api/solutions",
+        data={"title": "S", "problem_signature": "P", "root_cause": "R", "solution": "Do S"},
+        content_type="application/json",
+    ).json()
+
+    response = client.post(
+        f"/api/solutions/{solution['id']}/link-ticket",
+        data={"ticket_id": ticket.id, "link_type": "adopted"},
+        content_type="application/json",
+    )
+
+    assert response.status_code == 200
+    assert response.json()["link_type"] == "adopted"
+    assert response.json()["solution_usage_count"] == 1