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.

shopdb/plugins/base.py

@@ -205,15 +205,35 @@ class BasePlugin(ABC):
 
     def get_dashboard_widgets(self) -> List[Dict]:
         """
-        Return dashboard widget definitions.
+        Return dashboard card definitions.
 
-        Each widget: {
-            'name': str,
-            'component': str,  # Frontend component name
-            'endpoint': str,   # API endpoint for data
-            'size': str,       # 'small', 'medium', 'large'
-            'position': int    # Order on dashboard
+        DATA AND SHAPE, not a component name. The card is drawn by one of core's
+        generic renderers; the plugin says what to show, where it comes from and
+        how to link it.
+
+        Contract 0.19.0 replaced a shape that named a Vue component per widget.
+        That could not survive a lean build - a plugin's component may never be
+        staged into the frontend bundle (ADR-013) - and in practice five plugins
+        declared widgets pointing at components nobody had written, which
+        rendered as nothing at all. It is the same correction ADR-010 already
+        made for asset panels.
+
+        Each card: {
+            'id': str,          # stable, unique across plugins
+            'title': str,       # heading
+            'endpoint': str,    # API path the card fetches its own data from
+            'render': str,      # core renderer: 'exceptions' | 'list' | 'stat'
+            'severity': str,    # 'critical' | 'warning' | 'info' - orders cards
+            'permission': str,  # card is hidden without it
+            'empty': str,       # 'hide' when nothing to report, or empty text
+            'position': int,    # order within a severity
+            'viewall': str,     # optional route behind the card heading
+            'map': dict,        # optional field mapping for the renderer,
+                                # e.g. {'title': 'hostname', 'detail': 'entryname'}
         }
+
+        `empty: 'hide'` is not cosmetic. A card that reports "nothing wrong"
+        every day teaches people to stop reading the page.
         """
         return []