harden(offer): escape admin product titles against HTML/markdown injection

The offer price list projects the admin-entered product title into a markdown
table cell that is rendered, unsanitised, into the public /offer/ page. Escape
the title at the projection boundary so it renders as literal text: HTML
metacharacters become entities and the markdown table pipe and link brackets are
escaped, so a title can neither inject markup nor form a javascript: link. The
committed offer prose keeps its trusted-content treatment (code-reviewed, not
runtime input).

backend/internal/payments/offer_test.go

@@ -60,6 +60,30 @@ func TestProjectOfferPricingMissingRailAndEscaping(t *testing.T) {
 	}
 }
 
+// TestProjectOfferPricingEscapesHTMLAndLinks checks an admin title carrying HTML or a markdown link
+// is neutralised so it cannot inject markup into the public offer: the tag becomes entities and the
+// link brackets are escaped (so no "javascript:" anchor forms). The raw metacharacters must not
+// survive into the projected markdown.
+func TestProjectOfferPricingEscapesHTMLAndLinks(t *testing.T) {
+	entries := []catalogEntry{{
+		id:     uuid.New(),
+		title:  `<script>alert(1)</script> [x](javascript:alert(2)) & "q"`,
+		atoms:  []atomQty{{atomType: "hints", quantity: 1}},
+		prices: []priceRow{{method: "", currency: CurrencyChip, amount: 5}},
+	}}
+	md := projectOfferPricing(entries)
+	for _, bad := range []string{"<script>", "</script>", "[x]", `& "q"`} {
+		if strings.Contains(md, bad) {
+			t.Errorf("unescaped %q survived into the projection:\n%s", bad, md)
+		}
+	}
+	for _, want := range []string{"&lt;script&gt;", `\[x\]`, "&amp;", "&quot;q&quot;"} {
+		if !strings.Contains(md, want) {
+			t.Errorf("want escaped %q in:\n%s", want, md)
+		}
+	}
+}
+
 // TestProjectOfferPricingEmpty checks an empty catalog projects to the empty string (no stray table
 // headers), so the offer's pricing marker is replaced with nothing.
 func TestProjectOfferPricingEmpty(t *testing.T) {