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.go

@@ -13,6 +13,15 @@ import (
 	"git.anthrove.art/public/go-fa-api/internal/urls"
 )
 
+// CategorizedTags groups FA's prefixed system tags by category. Names are
+// stored without their prefix (e.g. "s_hybrid_species" → Species "hybrid_species").
+type CategorizedTags struct {
+	Species    []string
+	Characters []string
+	Artists    []string
+	Types      []string
+}
+
 // Submission is a fully resolved FA submission as seen on /view/{id}/.
 type Submission struct {
 	ID              SubmissionID
@@ -27,6 +36,10 @@ type Submission struct {
 	Description     string // raw HTML; sanitise before rendering to a browser
 	DescriptionText string // plaintext convenience
 	Tags            []string
+	// CategorizedTags groups FA's prefixed system tags by category.
+	// FA emits these as tag-block entries inside div.submission-tags with
+	// prefixes s_ (species), c_ (character), a_/u_ (artist), and t_ (type).
+	CategorizedTags CategorizedTags
 	FileURL         string // absolute CDN URL; pass to Download
 	ThumbURL        string
 	Width           int // 0 if unknown / non-image