history: flag rows whose slug no longer means the same anime

Provider switch (anidb -> hianime-first) left anidb-era rows whose slugs
200 on hianime as a DIFFERENT anime: /watch/jujutsu-kaisen-2552 is now
Burning Kabaddi (the trailing number is a hianime id), and the stored
anidb ep id doesn't even exist in that anime's list — the row rendered
as watched JJK but linked to the wrong show.

Client-side validation (no data migration): every history row's slug is
checked against GET /api/anime/{slug} once per slug; title mismatch (or
a dead slug) marks the entry stale. Stale rows:
- home: one card per stale anime above the hero + 'history ->' link
- history page: separate 'Moved or renamed on the source' section
Each card: title, 'find it ->' (search by stored title) and x (remove,
same endpoint as regular cards). Current rows are untouched: a hianime
slug whose details name matches the stored title passes.

Tests: API contract for a stale row (deletable while on the board);
live browser test (gated) with the real jujutsu-kaisen-2552 row:
remap card renders, card doesn't link to the wrong slug, find-it
lands on the title's search.

tests/test_api.py

@@ -506,3 +506,25 @@ def test_fetch_parallel_stitches_range_lanes():
     finally:
         u._session = real
         u._get = real_get
+
+
+def test_stale_entries_surfaced_as_remappable(base=BASE):
+    # provider switch left anidb-era rows: their slug 200s on hianime as a
+    # DIFFERENT anime (id 2552 = Burning Kabaddi). The client flags the
+    # mismatch; the API contract is unchanged — the entry is deletable by
+    # slug+ep while it sits on the remap board.
+    jar = login("bob", "bobpw12345", base)
+    stale = {"title": "Jujutsu Kaisen", "slug_id": "jujutsu-kaisen-2552",
+             "season_id": 2552, "ep_id": 35996, "lang": "jpn", "ep": 1, "pos": 42}
+    good = {"title": "Seed Anime", "slug_id": "seed-1", "season_id": 1,
+            "ep_id": 1, "lang": "eng", "ep": 1, "pos": 10}
+    assert req("POST", "/api/history", stale, jar=jar, base=base) == 200
+    assert req("POST", "/api/history", good, jar=jar, base=base) == 200
+    all_ = _json("GET", "/api/history?all=1", jar=jar, base=base)
+    got = [x for x in all_ if x["slug_id"] == "jujutsu-kaisen-2552"]
+    assert got and got[0]["ep_id"] == 35996 and got[0]["pos"] == 42, got
+    removed = _json("DELETE", "/api/history?slug_id=jujutsu-kaisen-2552&ep_id=35996",
+                    jar=jar, base=base)
+    assert removed["removed"] >= 1, removed
+    all_ = _json("GET", "/api/history?all=1", jar=jar, base=base)
+    assert not [x for x in all_ if x["slug_id"] == "jujutsu-kaisen-2552"], all_