feat: retire closed issues during Following review (Closes #1305)

src/main.py

@@ -3738,7 +3738,7 @@ async def global_search_preview(
 
 
 async def _search_preview_subscription_target(
-    owner: str, repo: str, number: int, kind: str
+    owner: str, repo: str, number: int, kind: str, *, allow_closed: bool = False
 ) -> tuple[str, dict]:
     repository = f"{owner}/{repo}"
     preview = await gitea_proxy.work_preview(repository, kind, number)
@@ -3747,7 +3747,7 @@ async def _search_preview_subscription_target(
         or preview.get("repository") != repository
         or preview.get("kind") != "issue"
         or preview.get("number") != number
-        or preview.get("state") != "open"
+        or preview.get("state") not in ({"open", "closed"} if allow_closed else {"open"})
     ):
         raise HTTPException(status_code=404, detail="Watchable search result not found")
     return repository, preview
@@ -3790,7 +3790,9 @@ async def mutate_global_search_preview_subscription(
 ) -> JSONResponse:
     watching = request.method == "PUT"
     try:
-        repository, preview = await _search_preview_subscription_target(owner, repo, number, kind)
+        repository, preview = await _search_preview_subscription_target(
+            owner, repo, number, kind, allow_closed=not watching
+        )
         login = await _confirmed_login()
         store = _following_store()
         following_item = {