Unify JSON error responses into one canonical shape

All JSON error responses now use a single format built by the new
datasette.utils.error_body() helper:

    {"ok": false, "error": "...", "errors": ["..."], "status": 400}

- error is all messages joined with '; ', errors is the full list,
  status always matches the HTTP status code
- The exception handler no longer emits the legacy title key in JSON
  (it is still available to the HTML error template)
- The permission debug endpoints (/-/allowed, /-/rules, /-/check,
  POST /-/permissions) no longer return bare {"error": ...} objects
- JSON renderer SQL errors keep their rows/truncated context keys but
  now include the canonical keys as well
- _shape=object misuse (queries or tables without primary keys) now
  returns HTTP 400 instead of 200 with an error body
- Method-not-allowed 405 responses use the canonical shape

Adds tests/test_error_shape.py covering all four previous shape
producers, updates affected tests, and documents the format in a new
'Error responses' section of docs/json_api.rst.

Implements section 1 of stable-api-recommendations.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GrHZSypDfMnym1tM5XJAFZ

tests/test_base_view.py

@@ -53,6 +53,8 @@ async def test_get_view():
     assert json.loads(post_json_response.body) == {
         "ok": False,
         "error": "Method not allowed",
+        "errors": ["Method not allowed"],
+        "status": 405,
     }
     assert post_json_response.status == 405