fix: handle invalid date formats gracefully and add missing dialog descriptions

- Wrap date.fromisoformat() in try/except in case_update tool — prevents
  unhandled ValueError from surfacing as 500; FastAPI now catches it as 422
- Add DialogDescription (sr-only) to 5 dialogs missing aria-describedby:
  documents-panel preview + delete, drafts-panel delete + feedback, link-related-dialog

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/app.py

@@ -1378,21 +1378,24 @@ async def api_case_update(case_number: str, req: CaseUpdateRequest, background_t
     existing = await db.get_case_by_number(case_number)
     old_status = (existing or {}).get("status", "")
 
-    result = await cases_tools.case_update(
-        case_number=case_number,
-        status=req.status,
-        title=req.title,
-        subject=req.subject,
-        notes=req.notes,
-        hearing_date=req.hearing_date,
-        decision_date=req.decision_date,
-        tags=req.tags,
-        expected_outcome=req.expected_outcome,
-        appellants=req.appellants,
-        respondents=req.respondents,
-        property_address=req.property_address,
-        permit_number=req.permit_number,
-    )
+    try:
+        result = await cases_tools.case_update(
+            case_number=case_number,
+            status=req.status,
+            title=req.title,
+            subject=req.subject,
+            notes=req.notes,
+            hearing_date=req.hearing_date,
+            decision_date=req.decision_date,
+            tags=req.tags,
+            expected_outcome=req.expected_outcome,
+            appellants=req.appellants,
+            respondents=req.respondents,
+            property_address=req.property_address,
+            permit_number=req.permit_number,
+        )
+    except ValueError as exc:
+        raise HTTPException(422, str(exc))
     try:
         parsed = json.loads(result)
     except json.JSONDecodeError: