feat(submission): parse FA's prefixed system tags into CategorizedTags

FA renders its species/character/artist/type system tags as tag-block
anchors with a data-tag-name carrying a single-letter prefix
(s_/c_/a_-u_/t_) and a sibling tag-invalid span instead of a /search/
link. The existing keyword pass skips them, so they were lost.

Adds a Submission.CategorizedTags field exposing the four buckets with
the prefix stripped, plus an examples/categorized_tags runnable demo.

submission_parser.go

@@ -156,6 +156,32 @@ func parseSubmission(id SubmissionID, doc *goquery.Document) (*Submission, error
 		}
 	})
 
+	// Prefixed system tags FA renders these as tag-block anchors with a
+	// data-tag-name attribute carrying a leading single-letter prefix:
+	//   s_ species, c_ character, a_/u_ artist, t_ type.
+	// They are paired with a sibling <span class="tag-invalid"> and have no
+	// /search/ href, so they are skipped by the keyword pass above.
+	doc.Find("div.submission-tags a.tag-block[data-tag-name]").Each(func(_ int, a *goquery.Selection) {
+		raw := strings.TrimSpace(trimAttr(a, "data-tag-name"))
+		if len(raw) < 3 || raw[1] != '_' {
+			return
+		}
+		name := raw[2:]
+		if name == "" {
+			return
+		}
+		switch raw[0] {
+		case 's':
+			s.CategorizedTags.Species = append(s.CategorizedTags.Species, name)
+		case 'c':
+			s.CategorizedTags.Characters = append(s.CategorizedTags.Characters, name)
+		case 'a', 'u':
+			s.CategorizedTags.Artists = append(s.CategorizedTags.Artists, name)
+		case 't':
+			s.CategorizedTags.Types = append(s.CategorizedTags.Types, name)
+		}
+	})
+
 	// File URL FA renders a "Download" button in #submission-options that
 	// links to the canonical file for *every* submission type. For visual
 	// art it equals the #submissionImg source; for stories and music it's