Serve an uploaded file as data, not as a document that can run

An SVG is an XML document that may carry a script, and it is an accepted image
type because floor-plan maps and branding genuinely want vector. Loaded through
an img tag that script never runs, so the tiles and maps were never the risk.
Opening the file's own URL is - and the application image route is public, so
that URL needs no session.

Every route that serves an upload now goes through one helper that sends
Content-Security-Policy: default-src 'none'; sandbox, and nosniff. Seven routes
across core and five plugins, so a new one added later starts from the same
place rather than repeating the reasoning. Banning the format instead would
have cost the maps their only sensible one.

The app also sent no security headers at all. It now sets nosniff,
frame-ancestors self (as X-Frame-Options too, for the display bays' browsers)
and a referrer policy. Deliberately NOT a page-wide CSP: this serves an SPA with
inline styles, so a real script-src policy is a change worth making with the
frontend in front of you, and a permissive header claiming one would be worse
than having none.

Contract 0.19.0. send_upload is on the shopdb.api surface, because a plugin
serving user-supplied bytes should not have to remember these headers. The same
bump records that get_dashboard_widgets has taken data and shape rather than a
component name since the dashboard was rebuilt - that shipped without a bump,
while BasePlugin and PLUGIN-HOOKS.md both still documented the shape nothing
renders, which is how five plugins came to declare widgets pointing at
components nobody had written.

tests/test_core/test_security_headers.py

@@ -0,0 +1,57 @@
+"""Responses carry the baseline headers, and uploads carry the strict ones.
+
+The application image route is PUBLIC and one accepted image type is an XML
+document that can carry a script. Loaded through an <img> tag that script never
+runs, so the tiles were never the risk; opening the file's own URL is. These
+tests pin the headers that make that case inert, because the failure is silent -
+nothing about the page looks different when they are missing.
+"""
+
+import io
+
+import pytest
+
+from shopdb.utils.uploads import UPLOAD_CSP
+
+
+def test_every_response_carries_the_baseline_headers(client):
+    resp = client.get('/api/health')
+    assert resp.headers.get('X-Content-Type-Options') == 'nosniff'
+    assert resp.headers.get('X-Frame-Options') == 'SAMEORIGIN'
+    assert resp.headers.get('Referrer-Policy') == 'strict-origin-when-cross-origin'
+
+
+def test_an_error_response_carries_them_too(client):
+    """after_request runs for error handlers as well, which is where a response
+    built by a different code path would otherwise slip through."""
+    resp = client.get('/api/no-such-route')
+    assert resp.status_code == 404
+    assert resp.headers.get('X-Content-Type-Options') == 'nosniff'
+
+
+def test_a_served_upload_is_sandboxed(client, app, tmp_path):
+    """The uploaded file itself gets a policy that blocks script execution."""
+    from shopdb.core.api.applications import _appimage_dir
+
+    with app.app_context():
+        directory = _appimage_dir()
+    svg = ('<svg xmlns="http://www.w3.org/2000/svg">'
+           '<script>window.top.location="http://evil"</script></svg>')
+    path = directory / 'probe-headers.svg' if hasattr(directory, '__truediv__') \
+        else None
+    if path is None:
+        import os
+        path = os.path.join(directory, 'probe-headers.svg')
+        with open(path, 'w') as handle:
+            handle.write(svg)
+    else:
+        path.write_text(svg)
+
+    try:
+        resp = client.get('/api/applications/image/probe-headers.svg')
+        assert resp.status_code == 200
+        assert resp.headers.get('Content-Security-Policy') == UPLOAD_CSP
+        assert resp.headers.get('X-Content-Type-Options') == 'nosniff'
+    finally:
+        import os
+        os.remove(path)