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.

web/static/app.css

@@ -129,6 +129,11 @@ body {
 }
 .code .gutter { flex-shrink: 0; }
 .codebody { padding: 0 18px; white-space: pre; }
+/* syntax highlight tokens (#1) */
+.tok-kw { color: #c792ea; }
+.tok-str { color: #a5e075; }
+.tok-num { color: #f78c6c; }
+.tok-com { color: #6a737d; font-style: italic; }
 .footnote { display: flex; gap: 20px; padding: 10px 18px; font-size: 20.7px; color: var(--muted-fg); border-top: 1px solid var(--border); flex-wrap: wrap; }
 
 /* history */