fix: move coerce_value() method to base GridFilter class

this should be standard practice for non-alchemy filters too

src/wuttaweb/grids/filters.py

@@ -350,6 +350,26 @@ class GridFilter:  # pylint: disable=too-many-instance-attributes
 
         return normalized
 
+    def coerce_value(self, value):
+        """
+        Coerce the given value to the correct type/format for use with
+        the filter.  This is where e.g. a boolean or date filter
+        should convert input string to ``bool`` or ``date`` value.
+
+        This is (usually) called from a filter method, when applying
+        the filter.  See also :meth:`apply_filter()`.
+
+        Default logic on the base class returns value as-is; subclass
+        may override as needed.
+
+        :param value: Input string provided by the user via the filter
+           form submission.
+
+        :returns: Value of the appropriate type, depending on the
+           filter subclass.
+        """
+        return value
+
     def apply_filter(self, data, verb=None, value=UNSPECIFIED):
         """
         Filter the given data set according to a verb/value pair.
@@ -420,15 +440,6 @@ class AlchemyFilter(GridFilter):
             if len(columns) == 1:
                 self.nullable = columns[0].nullable
 
-    def coerce_value(self, value):
-        """
-        Coerce the given value to the correct type/format for use with
-        the filter.
-
-        Default logic returns value as-is; subclass may override.
-        """
-        return value
-
     def filter_equal(self, query, value):
         """
         Filter data with an equal (``=``) condition.