feat: add JSON/JSONB sub-field select, filter and sort

Support column references that traverse into JSON/JSONB values —
data->>'x', data#>>'{a,b}', data->'a'->>'b', and the dotted data.a.b
shorthand — in SELECT column lists, WHERE filters and ORDER BY, across
the restheadspec, resolvespec, websocketspec and mqttspec handlers.

- pkg/common/json_column.go: canonical ParseColumnRef + ColumnRef.SQL()
  builder. JSON path segments are bound as a single ?::text[] parameter,
  never interpolated; cast targets are whitelisted via NormalizeCastTarget.
- pkg/common/json_condition.go: shared entry points mirroring
  BuildSpatialCondition - ResolveJSONColumnExpr (select/sort),
  BuildJSONFilterCondition (where, full operator set; infers ::numeric for
  ordered comparisons on numeric values when no explicit cast is given),
  and the ApplySelectColumns helper.
- pkg/reflection.IsJSONColumn / pkg/spectypes.IsJSONType: disambiguate the
  dotted shorthand (data.city is JSON only when the base is a JSON column).
- pkg/common/validation.go: ColumnValidator accepts JSON tokens.
- Handlers: thread model through the filter call chains and wire the
  select/sort paths.

funcspec (raw-SQL string builder, no param binding or model) and the
FetchRowNumber raw-SQL builders are left as follow-ups, as is OpenAPI
reporting of JSON sub-field columns.

pkg/mqttspec/handler.go

@@ -676,7 +676,7 @@ func (h *Handler) readByID(hookCtx *HookContext) (interface{}, error) {
 
 	// Apply columns
 	if hookCtx.Options != nil && len(hookCtx.Options.Columns) > 0 {
-		query = query.Column(hookCtx.Options.Columns...)
+		query = common.ApplySelectColumns(query, hookCtx.Model, "", hookCtx.Options.Columns)
 	}
 
 	// Apply preloads (simplified)
@@ -714,6 +714,10 @@ func (h *Handler) readMultiple(hookCtx *HookContext) (data interface{}, metadata
 	if hookCtx.Options != nil {
 		// Apply filters
 		for _, filter := range hookCtx.Options.Filters {
+			if cond, jargs, ok := common.BuildJSONFilterCondition(hookCtx.Model, "", filter.Column, filter.Operator, filter.Value); ok {
+				query = query.Where(cond, jargs...)
+				continue
+			}
 			op := strings.ToLower(filter.Operator)
 			if op == "like" || op == "ilike" {
 				query = query.Where(fmt.Sprintf("CAST(%s AS TEXT) %s ?", filter.Column, h.getOperatorSQL(filter.Operator)), filter.Value)
@@ -728,6 +732,10 @@ func (h *Handler) readMultiple(hookCtx *HookContext) (data interface{}, metadata
 			if sort.Direction == "desc" {
 				direction = "DESC"
 			}
+			if expr, jargs, _, ok := common.ResolveJSONColumnExpr(hookCtx.Model, "", sort.Column); ok {
+				query = query.OrderExpr(fmt.Sprintf("%s %s", expr, direction), jargs...)
+				continue
+			}
 			query = query.Order(fmt.Sprintf("%s %s", sort.Column, direction))
 		}
 
@@ -746,7 +754,7 @@ func (h *Handler) readMultiple(hookCtx *HookContext) (data interface{}, metadata
 
 		// Apply columns
 		if len(hookCtx.Options.Columns) > 0 {
-			query = query.Column(hookCtx.Options.Columns...)
+			query = common.ApplySelectColumns(query, hookCtx.Model, "", hookCtx.Options.Columns)
 		}
 	}
 
@@ -772,6 +780,10 @@ func (h *Handler) readMultiple(hookCtx *HookContext) (data interface{}, metadata
 	countQuery := h.db.NewSelect().Model(hookCtx.ModelPtr).Table(hookCtx.TableName)
 	if hookCtx.Options != nil {
 		for _, filter := range hookCtx.Options.Filters {
+			if cond, jargs, ok := common.BuildJSONFilterCondition(hookCtx.Model, "", filter.Column, filter.Operator, filter.Value); ok {
+				countQuery = countQuery.Where(cond, jargs...)
+				continue
+			}
 			op := strings.ToLower(filter.Operator)
 			if op == "like" || op == "ilike" {
 				countQuery = countQuery.Where(fmt.Sprintf("CAST(%s AS TEXT) %s ?", filter.Column, h.getOperatorSQL(filter.Operator)), filter.Value)