feat(web): parse YouTube video id from pasted URL forms

Pure parser for watch?v=, youtu.be/, shorts/, embed/, and bare ids; rejects
non-YouTube hosts and malformed input. Foundation for paste-a-URL summarize
(Feature 2). No fetch, no gate interaction — parsing only.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/web/paste.go

@@ -0,0 +1,62 @@
+package web
+
+import (
+	"fmt"
+	"net/url"
+	"regexp"
+	"strings"
+)
+
+// youtubeVideoID matches a canonical YouTube video id: exactly 11 URL-safe chars.
+var youtubeVideoID = regexp.MustCompile(`^[A-Za-z0-9_-]{11}$`)
+
+// parseYouTubeVideoID extracts the 11-character video id from a pasted YouTube
+// URL (watch?v=, youtu.be/, shorts/, embed/) or a bare id. It rejects non-YouTube
+// hosts and anything that doesn't yield a valid id, so the paste flow never tries
+// to fetch a video that can't exist (Feature 2).
+func parseYouTubeVideoID(raw string) (string, error) {
+	s := strings.TrimSpace(raw)
+	if s == "" {
+		return "", fmt.Errorf("empty input")
+	}
+
+	// Bare id (no URL) — accept directly.
+	if youtubeVideoID.MatchString(s) {
+		return s, nil
+	}
+
+	// Accept scheme-less URLs (youtube.com/watch?v=...) by giving url.Parse a host.
+	if !strings.Contains(s, "://") {
+		s = "https://" + s
+	}
+	u, err := url.Parse(s)
+	if err != nil {
+		return "", fmt.Errorf("not a URL: %w", err)
+	}
+
+	host := strings.ToLower(u.Hostname())
+	isYouTube := host == "youtu.be" || host == "youtube.com" || strings.HasSuffix(host, ".youtube.com")
+	if !isYouTube {
+		return "", fmt.Errorf("not a YouTube URL: %q", host)
+	}
+
+	var id string
+	switch {
+	case host == "youtu.be":
+		// youtu.be/<id>
+		id = strings.Trim(u.Path, "/")
+	case u.Path == "/watch":
+		id = u.Query().Get("v")
+	default:
+		// /shorts/<id>, /embed/<id>
+		parts := strings.Split(strings.Trim(u.Path, "/"), "/")
+		if len(parts) == 2 && (parts[0] == "shorts" || parts[0] == "embed") {
+			id = parts[1]
+		}
+	}
+
+	if !youtubeVideoID.MatchString(id) {
+		return "", fmt.Errorf("no YouTube video id in %q", raw)
+	}
+	return id, nil
+}