Bound title and language at create time (#86)

Titles are trimmed and truncated to 200 characters; language must be
blank or match ^[a-zA-Z0-9+#-]{1,40}$ (max 40 chars) or the create is
rejected with a clear 400. Regression tests cover truncation, malformed
and oversized language values, valid identifiers, and blank metadata.

internal/api/server.go

@@ -165,6 +165,27 @@ func (a *apiServer) handleCreatePaste(w http.ResponseWriter, r *http.Request) {
 		writeErr(w, status, msg)
 		return
 	}
+	// #86: bound free-form metadata at create time
+	if p.Title != nil {
+		t, err := checkTitle(*p.Title)
+		if err != nil {
+			writeErr(w, 400, err.Error())
+			return
+		}
+		p.Title = &t
+	}
+	if p.Language != nil {
+		l, err := checkLanguage(*p.Language)
+		if err != nil {
+			writeErr(w, 400, err.Error())
+			return
+		}
+		if l == "" {
+			p.Language = nil
+		} else {
+			p.Language = &l
+		}
+	}
 	if p.BurnAfterReads != nil { // #68: reject negative read budgets
 		if err := parseBurnAfterReads(*p.BurnAfterReads); err != nil {
 			writeErr(w, 400, err.Error())