feat: 404 log and report endpoint

- POST /_seo/api/v1/notfound with the golem15.seo.notfound_token bearer
  (constant-time, off when unset), URL and referer caps, a row cap with
  an hourly warning, throttle bucket seo-notfound
- golem15.seo.notfound list by hits with Resolved rows, a preview with the
  resolved hint, a prompted 301 and a 410 record action, a bulk 410 that
  refuses a selection with a covered address
- every /_seo/api/v1 answer carries X-Robots-Tag: noindex

models/notfound.go

@@ -0,0 +1,65 @@
+package models
+
+import (
+	"time"
+
+	"gorm.io/gorm"
+)
+
+// NotFound is one address of the 404 log on golem15_seo_notfound (D-20):
+// the path (with its query) that answered 404, how often, when first and
+// last, and the referer of the last hit. The frontend reports each 404
+// through the authenticated report endpoint.
+type NotFound struct {
+	ID        uint       `gorm:"column:id;primaryKey"`
+	URL       string     `gorm:"column:url"`
+	Hits      int64      `gorm:"column:hits"`
+	FirstSeen *time.Time `gorm:"column:first_seen"`
+	LastHit   *time.Time `gorm:"column:last_hit"`
+	Referer   string     `gorm:"column:referer"`
+}
+
+func (NotFound) TableName() string { return "golem15_seo_notfound" }
+
+// MorphName names a 404 log entry in history.
+func (NotFound) MorphName() string { return `Golem15\SEO\Models\NotFound` }
+
+// Fillable is empty: entries come from the report endpoint only, and the
+// preview edits nothing.
+func (NotFound) Fillable() []string { return []string{} }
+
+// Rules is empty: nothing is edited.
+func (NotFound) Rules() map[string]string { return map[string]string{} }
+
+// The 404 log's status filter scope and its values.
+const (
+	FilterNotFoundStatus = "FilterNotFoundStatus"
+	NotFoundOpen         = "open"
+	NotFoundResolved     = "resolved"
+)
+
+// FilterScopes lists the 404 log's filter scopes.
+func (NotFound) FilterScopes() []string { return []string{FilterNotFoundStatus} }
+
+// coveredSQL is true for an entry an exact redirect or 410 answers now
+// (the list filter; the Resolved badge also counts patterns and regex).
+const coveredSQL = `EXISTS (SELECT 1 FROM golem15_seo_redirects AS r WHERE r.match = 'exact' AND (r.source = golem15_seo_notfound.url OR (r.ignore_case AND lower(r.source) = lower(golem15_seo_notfound.url))))`
+
+// FilterScope narrows the 404 log to open or resolved entries; an unknown
+// value matches nothing.
+func (NotFound) FilterScope(name string, db *gorm.DB, value any) *gorm.DB {
+	if db == nil || name != FilterNotFoundStatus {
+		return db
+	}
+	switch s, _ := value.(string); s {
+	case NotFoundOpen:
+		return db.Where("NOT " + coveredSQL)
+	case NotFoundResolved:
+		return db.Where(coveredSQL)
+	}
+	return db.Where("1 = 0")
+}
+
+func init() {
+	Register(NotFound{})
+}

models/notfound/columns.yaml

@@ -0,0 +1,23 @@
+# 404 log columns (UI-SPEC C9).
+columns:
+    url:
+        label: golem15.seo::lang.notfound.url
+        type: text
+        searchable: true
+        sortable: true
+    hits:
+        label: golem15.seo::lang.notfound.hits
+        type: text
+        sortable: true
+    last_hit:
+        label: golem15.seo::lang.notfound.last_hit
+        type: datetime
+        sortable: true
+    referer:
+        label: golem15.seo::lang.notfound.referer
+        type: text
+    first_seen:
+        label: golem15.seo::lang.notfound.first_seen
+        type: datetime
+        sortable: true
+        invisible: true

models/notfound/fields.yaml

@@ -0,0 +1,29 @@
+# 404 log preview (UI-SPEC C9): every field is read-only.
+fields:
+    url:
+        label: golem15.seo::lang.notfound.url
+        type: text
+        span: full
+        context: preview
+    hits:
+        label: golem15.seo::lang.notfound.hits
+        type: number
+        span: left
+        context: preview
+    last_hit:
+        label: golem15.seo::lang.notfound.last_hit
+        type: datepicker
+        mode: datetime
+        span: right
+        context: preview
+    first_seen:
+        label: golem15.seo::lang.notfound.first_seen
+        type: datepicker
+        mode: datetime
+        span: left
+        context: preview
+    referer:
+        label: golem15.seo::lang.notfound.referer
+        type: text
+        span: right
+        context: preview