Answer a nonsense filter with a 400, and sweep every list route for the rest

Wave 3. Six findings, one cause: `int(request.args.get('typeid'))` with nothing
catching the ValueError. The caller sent a bad argument and got
500 INTERNAL_ERROR plus a traceback in the log, which says the server is broken
when the truth is that 'abc' is not a number.

shopdb/utils/queryargs.intarg reads it and raises QueryArgError, which an
app-level handler answers as a 400 naming the argument. App-level on purpose: a
route should not have to remember, and forgetting was the entire defect. The
policy differs from get_pagination_params, which falls back to a default for a
bad page number - a nonsense page is harmless and the caller still wants their
list, but silently ignoring a nonsense FILTER returns rows they believe they
excluded.

The gate matters more than the six fixes. tests/test_core/test_bad_query_args.py
sweeps all 131 registered no-parameter GET routes under /api/ with twelve
argument shapes and fails on any 5xx. It found sites the test run had not
reported - statusid and locationid on /api/assets, the second asset-list route,
limit on the employee lookup and the printer supply history - because the
pattern is how a list filter gets written rather than a property of any one
route. Verified non-vacuous by planting the old `int(...)` back into
/api/machines: four tests fail.

Four usb routes are skipped by exact path, with the reason: they reach the
EXTERNAL cmmc_usb database, so their 500 in a test environment is a missing
database rather than an argument fault. That is the complete set - every usb
GET route calling cmmc_usb_connection - not a convenient subset.

One correction worth recording: the first pass rewrote every
`if x := request.args.get(...)` in the touched files, including text arguments,
so a search for 'ZZNOTHINGMATCHES' briefly became a 400. Caught by
test_list_search_identifiers. Only id-shaped arguments are integers.

Scope: product

plugins/knowledgebase/api/routes.py

@@ -4,6 +4,7 @@ from flask import Blueprint, request
 from flask_jwt_extended import jwt_required
 
 from shopdb.api import (
+    intarg,
     db,
     Application,
     success_response,
@@ -16,7 +17,7 @@ from shopdb.api import (
 
 from ..models import KnowledgeBase
 
-from shopdb.api import require_permission, apply_import_timestamps
+from shopdb.api import require_permission, apply_import_timestamps, intarg
 
 knowledgebase_bp = Blueprint('knowledgebase', __name__)
 
@@ -91,8 +92,8 @@ def list_articles():
         query = query.filter(_search_clause(search))
 
     # Filter by topic/application
-    if appid := request.args.get('appid'):
-        query = query.filter(KnowledgeBase.appid == int(appid))
+    if (appid := intarg('appid')) is not None:
+        query = query.filter(KnowledgeBase.appid == appid)
 
     # Exact-match natural-key lookups for idempotent import. linkurl is the
     # stable natural key; shortdescription (the title) is offered as a fallback.