feat: per-IP rate limit and write bans

Apply sliding-window 2/s limits and BannedIP checks on write routes.
Also force SVG attachment disposition and claim burn-after-read in DB
before streaming, deleting the file via BackgroundTask after the response.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/test_items.py

@@ -119,3 +119,83 @@ def test_file_burn_after_download_not_metadata(client):
 
     assert client.get(f"/api/items/{slug}/file").status_code == 404
     assert client.get(f"/api/items/{slug}").status_code == 404
+
+
+def test_svg_forced_attachment_disposition(client):
+    files = {"file": ("xss.svg", b"<svg xmlns='http://www.w3.org/2000/svg'></svg>", "image/svg+xml")}
+    r = client.post(
+        "/api/items/upload",
+        files=files,
+        data={"is_public": "true", "ttl": "24h"},
+    )
+    assert r.status_code == 200
+    slug = r.json()["slug"]
+    f = client.get(f"/api/items/{slug}/file")
+    assert f.status_code == 200
+    cd = f.headers.get("content-disposition", "")
+    assert "attachment" in cd
+    assert "inline" not in cd.split(";")[0]
+
+
+def test_burn_claims_before_stream(client, monkeypatch):
+    """burned=True is set before streaming, not only in the post-response BackgroundTask."""
+    import app.db as db_module
+    import app.api.public as public_api
+    from app.models import Item
+    from sqlalchemy import select
+
+    # Background only deletes bytes; claiming is done in the request handler.
+    monkeypatch.setattr(public_api, "_delete_file_path", lambda *_a, **_k: None)
+
+    files = {"file": ("once.txt", b"payload", "text/plain")}
+    r = client.post(
+        "/api/items/upload",
+        files=files,
+        data={"is_public": "true", "ttl": "never", "burn_after_read": "true"},
+    )
+    assert r.status_code == 200
+    slug = r.json()["slug"]
+
+    first = client.get(f"/api/items/{slug}/file")
+    assert first.status_code == 200
+    assert first.content == b"payload"
+
+    db = db_module.SessionLocal()
+    try:
+        item = db.scalar(select(Item).where(Item.slug == slug))
+        assert item is not None
+        assert item.burned is True
+    finally:
+        db.close()
+
+    assert client.get(f"/api/items/{slug}/file").status_code == 404
+
+
+def test_rate_limit_third_request_in_same_second(client, monkeypatch):
+    import app.config as config_module
+    from app.services import rate_limit as rate_limit_module
+
+    monkeypatch.setattr(config_module.settings, "rate_limit_per_second", 2)
+    rate_limit_module.reset()
+
+    r1 = client.post("/api/items", json={"body": "one"})
+    r2 = client.post("/api/items", json={"body": "two"})
+    r3 = client.post("/api/items", json={"body": "three"})
+    assert r1.status_code == 200
+    assert r2.status_code == 200
+    assert r3.status_code == 429
+
+
+def test_banned_ip_cannot_create(client):
+    import app.db as db_module
+    from app.models import BannedIP
+
+    db = db_module.SessionLocal()
+    try:
+        db.add(BannedIP(ip="testclient", reason="test"))
+        db.commit()
+    finally:
+        db.close()
+
+    r = client.post("/api/items", json={"body": "x"})
+    assert r.status_code == 403