refactor: retire the images tarball backup

Images and attachments are backed up off-site, one object per file, so the
tar+zstd of /images goes: its service and restore, both tasks, the trigger and
restore routes, the Keep last setting, the screen's Images section, and the
tests. Migration 0122 drops backup_run.tar_path and
import_settings.backup_images_keep_last_n and deletes any images runs.

The local database dump stays for an install with no off-site destination,
and the Backups card now says plainly that images and attachments are backed
up only off-site.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_api_system_backup.py

@@ -16,21 +16,20 @@ async def _seed_runs(db):
     now = datetime.now(UTC)
     for i in range(4):
         db.add(BackupRun(
-            kind="db" if i % 2 == 0 else "images",
+            kind="db" if i % 2 == 0 else "media",
             status="ok",
             tag=None,
             triggered_by="manual",
             started_at=now - timedelta(seconds=10 - i),
             finished_at=now - timedelta(seconds=9 - i),
             sql_path="/tmp/fake.sql" if i % 2 == 0 else None,
-            tar_path=None if i % 2 == 0 else "/tmp/fake.tar.zst",
             size_bytes=100,
             manifest={},
         ))
     await db.commit()
 
 
-# --- POST /db, /images ----------------------------------------------
+# --- POST /db ---------------------------------------------------------
 
 
 @pytest.mark.asyncio
@@ -72,14 +71,10 @@ async def test_post_db_rejects_tag_too_long(client):
 
 
 @pytest.mark.asyncio
-async def test_post_images_dispatches(client, monkeypatch):
-    dispatched = []
-    monkeypatch.setattr(
-        "backend.app.tasks.backup.backup_images_task.delay",
-        lambda **kw: dispatched.append(kw),
-    )
-    resp = await client.post("/api/system/backup/images", json={})
-    assert resp.status_code == 202
+async def test_the_images_tarball_is_gone(client):
+    assert (await client.post("/api/system/backup/images", json={})).status_code in (404, 405)
+    resp = await client.get("/api/system/backup/runs?kind=images")
+    assert resp.status_code == 400
 
 
 # --- GET /runs -------------------------------------------------------
@@ -103,10 +98,10 @@ async def test_runs_filter_by_kind_db(client, _seed_runs):
 
 
 @pytest.mark.asyncio
-async def test_runs_filter_by_kind_images(client, _seed_runs):
-    resp = await client.get("/api/system/backup/runs?kind=images")
+async def test_runs_filter_by_kind_media(client, _seed_runs):
+    resp = await client.get("/api/system/backup/runs?kind=media")
     body = await resp.get_json()
-    assert all(r["kind"] == "images" for r in body["runs"])
+    assert body["runs"] and all(r["kind"] == "media" for r in body["runs"])
 
 
 @pytest.mark.asyncio
@@ -298,7 +293,7 @@ async def test_get_settings_returns_defaults(client):
     assert body["backup_db_nightly_enabled"] is False
     assert body["backup_db_nightly_hour_utc"] == 3
     assert body["backup_db_keep_last_n"] == 14
-    assert body["backup_images_keep_last_n"] == 3
+    assert "backup_images_keep_last_n" not in body
 
 
 @pytest.mark.asyncio