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.

plugins/printedparts/api/routes.py

@@ -236,8 +236,7 @@ def upload_item_image(item_id: int):
 @printedparts_bp.route('/image/<path:filename>', methods=['GET'])
 def serve_item_image(filename):
     """Serve an uploaded item image (public - kiosk and list read it)."""
-    from flask import send_from_directory
-    return send_from_directory(_imagedir(), filename)
+    return send_upload(_imagedir(), filename)
 
 
 @printedparts_bp.route('/items/<int:item_id>/image', methods=['DELETE'])
@@ -643,6 +642,7 @@ def report_by_person():
 # --- print files: append-only revisions per item ------------------------------
 
 from flask_jwt_extended import get_jwt_identity
+from shopdb.api import send_upload
 
 from ..models import PrintedItemFile
 
@@ -737,12 +737,11 @@ def upload_item_file(item_id: int):
 @jwt_required(optional=True)
 def download_item_file(file_id: int):
     """Download a revision under its original filename."""
-    from flask import send_from_directory
     record = db.session.get(PrintedItemFile, file_id)
     if not record:
         return error_response(ErrorCodes.NOT_FOUND, 'File not found',
                               http_code=404)
-    return send_from_directory(_filedir(), record.storedfilename,
+    return send_upload(_filedir(), record.storedfilename,
                                as_attachment=True,
                                download_name=record.filename)