v3: youtube embeds now honour the timestamp in the source url, with a migration backfilling existing dumps

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

api/services/providers/youtube.ts

@@ -32,6 +32,30 @@ function extractPlaylistId(url: string): string | null {
   }
 }
 
+/**
+ * Extract the playback offset in seconds from a YouTube URL.
+ * Accepts `t` (plain seconds, `90s`, or `1h2m3s` / `20m5s` forms) and `start`
+ * (plain seconds). Returns null when absent or unparseable.
+ */
+function extractStartSeconds(url: string): number | null {
+  let raw: string | null;
+  try {
+    const u = new URL(url);
+    raw = u.searchParams.get("t") ?? u.searchParams.get("start");
+  } catch {
+    return null;
+  }
+  if (!raw) return null;
+
+  const plain = /^\d+$/.exec(raw);
+  if (plain) return Number(raw);
+
+  const hms = /^(?:(\d+)h)?(?:(\d+)m)?(?:(\d+)s)?$/i.exec(raw);
+  if (!hms || (!hms[1] && !hms[2] && !hms[3])) return null;
+  return Number(hms[1] ?? 0) * 3600 + Number(hms[2] ?? 0) * 60 +
+    Number(hms[3] ?? 0);
+}
+
 /** Matches /channel/UC…, /@handle, /c/name, /user/name */
 function extractChannelPath(url: string): string | null {
   try {
@@ -134,6 +158,8 @@ export const youtubeProvider: RichContentProvider = {
 
     const embedParams = new URLSearchParams({ rel: "0" });
     if (listId) embedParams.set("list", listId);
+    const start = extractStartSeconds(url);
+    if (start) embedParams.set("start", String(start));
 
     return {
       type: "youtube",