Syntax highlighting, rate limiting, creator auto-unlock (#1, #2, #26)

#1: server-side regex highlighter (highlight.go) for go/python/js/json/bash/sql;
token span classes styled in app.css; per-line so gutter stays aligned.
#2: in-memory token-bucket rate limiter (ratelimit.go) on POST /api/pastes,
/api/guess-language and unlock POST; 429 + Retry-After + X-RateLimit headers.
#26: new-page JS POSTs the password to /{id} with ?next= after creation; the
unlock handler honors same-origin ?next= redirect so the creator lands on the
unlocked paste. POST /{id} route added.

Tests: ratelimit_test.go (burst/429, refill, unlock limit, highlight, auto-
unlock e2e); existing tests updated for per-test limiter isolation.

guess.go

@@ -62,6 +62,11 @@ func guessLang(s string) string {
 }
 
 func (a *apiServer) handleGuessLang(w http.ResponseWriter, r *http.Request) {
+	setRateLimitHeaders(w, 1, 5)
+	if !rateLimitGuess(r) {
+		writeRateLimited(w, 1)
+		return
+	}
 	var req struct {
 		Content string `json:"content"`
 	}