Answer API errors in the envelope, say when supplies are unread, badge each chain

Three more findings from the 2026-09-03 run.

Werkzeug answered 405, 415 and a malformed JSON body itself, in HTML. A client
written against the documented {status,data,meta} contract got a page of markup
with no error code, which is indistinguishable from the server having fallen
over. The 405 had a second edge: on a route the caller cannot otherwise see, a
method-not-allowed CONFIRMS the path exists, so a gated kiosk route meant to
read as absent announced itself by refusing the wrong verb. All three now
answer in the envelope, and only under /api/ - everything else is the SPA and
keeps the framework's behaviour, deep links included.

The Toner Report rendered "0 Printers Checked / 0 Low / 0 Critical" and "No
printers match the selected filter" while the API it had just called said
supply monitoring was unreachable. Zeros read as a clean bill of health for a
fleet nobody has looked at. The page now carries the reason the API gave and
suppresses the counts, because a number nobody measured is not a result. Same
defect shape as the supplies card fixed on 2026-08-29; `unavailable` was
already in the payload and the page ignored it.

Backup revisions badged "current" per (asset, kind). NTLARS settings describe a
MACHINE but are read off the controlling PC, so a machine with two reporting
PCs has two independent chains of the same kind - and only the higher revision
id was badged, so the second PC's newest revision showed as history and lost
its last-verified stamp. The machine looked like one source re-reporting rather
than two. sourcehostname is the chain; a revision without one forms its own,
which keeps the old behaviour for everything that has only ever had one source.

Nine tests. The envelope ones verified by planting: remove the three handlers
and three fail.

Scope: product

shopdb/__init__.py

@@ -435,6 +435,48 @@ def register_error_handlers(app: Flask):
             http_code=404
         )
 
+    # Werkzeug answers these three itself, in HTML, so a JSON client asking a
+    # question slightly wrong got a page of markup with no error code and no
+    # envelope - and on a route the caller cannot see, the 405 also confirmed
+    # that the path exists, which is how a gated kiosk route meant to look
+    # absent announced itself.
+    #
+    # Only under /api/. Everything else is the SPA, which needs the framework's
+    # own behaviour (and its own catch-all) left alone.
+    from flask import request as flask_request
+
+    def _api_request():
+        return flask_request.path.startswith('/api/')
+
+    @app.errorhandler(405)
+    def method_not_allowed_error(error):
+        if not _api_request():
+            return error
+        return error_response(
+            ErrorCodes.VALIDATION_ERROR,
+            'That method is not allowed on this endpoint.',
+            http_code=405)
+
+    @app.errorhandler(415)
+    def unsupported_media_type_error(error):
+        if not _api_request():
+            return error
+        return error_response(
+            ErrorCodes.VALIDATION_ERROR,
+            'This endpoint expects a JSON body; send '
+            'Content-Type: application/json.',
+            http_code=415)
+
+    @app.errorhandler(400)
+    def bad_request_error(error):
+        if not _api_request():
+            return error
+        # Werkzeug raises this for malformed JSON before any route sees it.
+        return error_response(
+            ErrorCodes.VALIDATION_ERROR,
+            'The request body could not be read as JSON.',
+            http_code=400)
+
     @app.errorhandler(500)
     def internal_error(error):
         return error_response(