fix(ui): history_diff 500s when a note's file vanished, not 404

svc.get_note(note_id) sat outside the try in history_diff, so a note
with history but no current file (unlinked without hard_delete) raised
an uncaught NotFound and returned 500 instead of 404. Moved it inside
the same try/except as the history-revision lookup. Audited the other
note_id routes: they already wrap their get_note/get calls in
try/except MdmemError, so this was the only route with the gap.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0139vHnNpS2sLNELjrAHX5Cu

src/mdmem/ui/app.py

@@ -322,9 +322,12 @@ def build_ui_app(svc: MemoryService) -> FastAPI:  # noqa: C901 - one place, many
     async def history_diff(request: Request, note_id: str, stamp: str) -> Response:
         try:
             old = svc.vault.read_history(note_id, stamp)
+            current = svc.get_note(note_id).to_text()
         except Exception as exc:
+            # A note can have history but no current file (unlinked outside
+            # hard_delete); svc.get_note then raises NotFound after the path
+            # validation above has already passed, so both must 404 here.
             return PlainTextResponse(str(exc), status_code=404)
-        current = svc.get_note(note_id).to_text()
         return templates.TemplateResponse(
             request,
             "partial_diff.html",

tests/test_ui_flows.py

@@ -124,6 +124,24 @@ async def test_note_page_shows_links_backlinks_and_history(ui):
     assert "llama-swap" in back.text
 
 
+async def test_history_diff_for_a_note_whose_file_vanished_returns_404(ui):
+    """A note can have history but no current file if it was unlinked outside of
+    hard_delete (e.g. by hand, or a bug elsewhere). svc.get_note then raises
+    NotFound — the route must turn that into 404, not an uncaught 500 (§10)."""
+    client, svc = ui
+    res = await svc.write_note(title="Verschwunden", body="v1", type="note")
+    await svc.edit(res["id"], body="v2", expected_version=svc.get_note(res["id"]).file_hash)
+    stamps = svc.vault.history(res["id"])
+    assert stamps, "editing an existing note must snapshot the previous version"
+
+    path = svc.vault.find(res["id"])
+    assert path is not None
+    path.unlink()
+
+    r = await client.get(f"/ui/n/{res['id']}/history/{stamps[0]}")
+    assert r.status_code == 404
+
+
 async def test_edit_conflict_shows_a_diff_and_keeps_both_versions(ui):
     client, svc = ui
     res = await svc.write_note(title="Konflikt", body="original", type="note")