Validate and normalize auto-postpone period to days (#2672)

* Validate and normalize auto-postpone period to days

- Add Entropy::AUTO_POSTPONE_PERIODS and validate auto_postpone_period against the allowed set
- Introduce auto_postpone_period_in_days for forms and entropy update endpoints
- Fall back to index 0 in knob partial when current value isn't in options
- Remove entropy_auto_close_options helper in favor of model constant
- Update tests to use allowed period values

* Use auto_postpone_period_in_days for JSON entropy updates

The JSON API was accepting auto_postpone_period (seconds) which
bypassed the days normalization and validation. Switch to
auto_postpone_period_in_days consistently and add explicit
wrap_parameters so flat JSON params are wrapped correctly for
the virtual attribute.

* Default to account or 30-day fallback when knob value is invalid

* Address PR review feedback for entropy validation

- Fall back to first knob option (index 0) when persisted value isn't in
  the allowed set, preventing nil index errors for legacy values
- Use integer division (1.day.to_i) for consistent day calculations

* Default to account entropy period instead of first option for knob fallback

* Test that default auto-postpone period is in the allowed periods

* Return 422 instead of 500 for invalid entropy auto-postpone values

Rescue ActiveRecord::RecordInvalid in entropy controllers so invalid
auto_postpone_period_in_days values return 422 Unprocessable Entity
instead of raising a 500.

* Fix NoMethodError when board entropy falls back to account default

Use container.account.entropy.auto_postpone_period_in_days instead of
container.account.auto_postpone_period_in_days since Account doesn't
delegate that method.

* Test board entropy fallback to account default for invalid periods

test/controllers/api/flat_json_params_test.rb

@@ -39,17 +39,17 @@ class FlatJsonParamsTest < ActionDispatch::IntegrationTest
   test "update board entropy with flat JSON" do
     board = boards(:writebook)
 
-    put board_entropy_path(board), params: { auto_postpone_period: 99.days }, as: :json
+    put board_entropy_path(board), params: { auto_postpone_period_in_days: 90 }, as: :json
 
     assert_response :no_content
-    assert_equal 99.days, board.entropy.reload.auto_postpone_period
+    assert_equal 90.days, board.entropy.reload.auto_postpone_period
   end
 
   test "update account entropy with flat JSON" do
-    put account_entropy_path, params: { auto_postpone_period: 2.days }, as: :json
+    put account_entropy_path, params: { auto_postpone_period_in_days: 7 }, as: :json
 
     assert_response :no_content
-    assert_equal 2.days, Current.account.entropy.reload.auto_postpone_period
+    assert_equal 7.days, Current.account.entropy.reload.auto_postpone_period
   end
 
   test "create push subscription with flat JSON" do