feat: pagination forms and suffixes in resolve and site

- Pagination tab: query or path form, per-locale segment, title and
  description suffixes with %page% and %pages%
- ?page=1, /<segment>/1 and the other form answer 301 to the configured
  address in one hop with normalisation and strict URLs; a page value
  that is not a number answers not_found with pagination.invalid
- page 2 and later: self canonical with the page, index,follow, the
  suffixes (%pages% left for the layer), paged hreflang alternates
- GET /site answers pagination {form, param, segments} and home_crumb

models/settings.go

@@ -51,8 +51,44 @@ const (
 	errHoursOrder            = "golem15.seo::lang.settings.hours_order"
 	errCountryInvalid        = "golem15.seo::lang.settings.country_invalid"
 	errCoordinateInvalid     = "golem15.seo::lang.settings.coordinate_invalid"
+	errPageSegmentInvalid    = "golem15.seo::lang.settings.page_segment_invalid"
 )
 
+// PagePlaceholders are the placeholders a page suffix may use (D-41).
+var PagePlaceholders = []string{"%page%", "%pages%"}
+
+// SuffixProblems refuses a page suffix with a placeholder other than
+// %page% and %pages%.
+func SuffixProblems(suffix string) []Problem {
+	for _, found := range placeholderPattern.FindAllString(suffix, -1) {
+		if !oneOfList(found, PagePlaceholders) {
+			return []Problem{{Key: errTemplateUnknown, Params: map[string]string{"placeholder": found}}}
+		}
+	}
+	return nil
+}
+
+// PaginationProblems lists the refusals of the Pagination tab: the form,
+// a segment outside [a-z0-9-] and unknown suffix placeholders.
+func (s Settings) PaginationProblems() map[string][]Problem {
+	out := map[string][]Problem{}
+	if s.PageForm != "" && !oneOfList(s.PageForm, []string{PageFormQuery, PageFormPath}) {
+		out["page_form"] = []Problem{{Key: errValueInvalid}}
+	}
+	if seg := strings.TrimSpace(s.PageSegment); seg != "" && !pageSegment.MatchString(seg) {
+		out["page_segment"] = []Problem{{Key: errPageSegmentInvalid}}
+	}
+	for field, v := range map[string]string{"page_title_suffix": s.PageTitleSuffix, "page_description_suffix": s.PageDescriptionSuffix} {
+		if p := SuffixProblems(v); p != nil {
+			out[field] = p
+		}
+	}
+	return out
+}
+
+// pageSegment is a valid page path segment.
+var pageSegment = regexp.MustCompile(`^[a-z0-9-]+$`)
+
 // LocalTypes are the LocalBusiness types the settings offer (UI-SPEC C20).
 var LocalTypes = []string{
 	"LocalBusiness", "ProfessionalService", "Store", "FoodEstablishment",
@@ -200,7 +236,8 @@ func (Settings) Fillable() []string {
 		"publisher_phone", "publisher_same_as", "local_enabled", "local_type",
 		"local_price_range", "local_street", "local_postal_code", "local_city",
 		"local_region", "local_country", "local_latitude", "local_longitude",
-		"local_hours",
+		"local_hours", "page_form", "page_segment", "page_title_suffix",
+		"page_description_suffix",
 	}
 }
 
@@ -304,6 +341,9 @@ func (s Settings) Problems() map[string][]Problem {
 			out[field] = p
 		}
 	}
+	for field, list := range s.PaginationProblems() {
+		out[field] = append(out[field], list...)
+	}
 	for field, list := range s.PublisherProblems() {
 		out[field] = append(out[field], list...)
 	}

models/settings/fields.yaml

@@ -4,7 +4,7 @@
 #
 # SEO settings (UI-SPEC C11, C20): General, one tab per content type (added
 # by the plugin's form extension after General), Indexing, Addresses,
-# Links, Publisher, Local business and Verification.
+# Pagination, Links, Publisher, Local business and Verification.
 
 fields:
     _staging:
@@ -146,6 +146,38 @@ fields:
         default: false
         tab: golem15.seo::lang.settings.tab_urls
 
+    page_form:
+        label: golem15.seo::lang.settings.page_form
+        comment: golem15.seo::lang.settings.page_form_comment
+        type: dropdown
+        span: left
+        default: query
+        options:
+            query: golem15.seo::lang.settings.page_form_query
+            path: golem15.seo::lang.settings.page_form_path
+        tab: golem15.seo::lang.settings.tab_pagination
+
+    page_segment:
+        label: golem15.seo::lang.settings.page_segment
+        comment: golem15.seo::lang.settings.page_segment_comment
+        type: mltext
+        span: right
+        tab: golem15.seo::lang.settings.tab_pagination
+
+    page_title_suffix:
+        label: golem15.seo::lang.settings.page_title_suffix
+        comment: golem15.seo::lang.settings.page_title_suffix_comment
+        type: mltext
+        span: left
+        tab: golem15.seo::lang.settings.tab_pagination
+
+    page_description_suffix:
+        label: golem15.seo::lang.settings.page_description_suffix
+        comment: golem15.seo::lang.settings.page_description_suffix_comment
+        type: mltext
+        span: right
+        tab: golem15.seo::lang.settings.tab_pagination
+
     links_new_tab:
         label: golem15.seo::lang.settings.links_new_tab
         comment: golem15.seo::lang.settings.links_new_tab_comment