feat(player): linkify descriptions and play YouTube links inline

- Narrow the title hover target to the actual text, not the whole row.
- Linkify descriptions: timestamps (mm:ss / hh:mm:ss) seek the player; emails
  become mailto:; hashtags link to YouTube's hashtag feed; other URLs open in a
  new tab. Blank lines are stripped so the popover isn't mostly whitespace.
- YouTube links play in the inline player: a link to the current video seeks
  (honoring t=), a link to another video navigates the player to it, with a Back
  button to the original. While on a linked video the title/author come from the
  player and its views/date/duration + a clickable channel come from the detail
  endpoint, which falls back to the YouTube API (videos.list, attributed to the
  user) for videos not in our DB.

backend/app/routes/feed.py

@@ -4,9 +4,12 @@ from fastapi import APIRouter, Depends, HTTPException, Query
 from sqlalchemy import Select, and_, false, func, or_, select
 from sqlalchemy.orm import Session, aliased
 
+from app import quota
 from app.auth import current_user
 from app.db import get_db
 from app.models import Channel, ChannelTag, Subscription, Tag, User, Video, VideoState
+from app.sync.videos import parse_iso8601_duration
+from app.youtube.client import YouTubeClient, YouTubeError
 
 router = APIRouter(prefix="/api", tags=["feed"])
 
@@ -296,12 +299,48 @@ def get_video_detail(
     db: Session = Depends(get_db),
 ) -> dict:
     """On-demand detail (description, like count) — kept out of the feed list payload
-    so the feed stays lean; fetched lazily, e.g. for the title hover popover."""
+    so the feed stays lean; fetched lazily, e.g. for the title hover popover.
+
+    Videos we already store are served from the DB for free. A video that isn't in
+    our DB (e.g. a YouTube link inside another video's description that the in-app
+    player navigated to) is resolved via the YouTube API (videos.list, 1 unit,
+    attributed to the requesting user)."""
     v = db.get(Video, video_id)
-    if v is None:
+    if v is not None:
+        return {
+            "id": v.id,
+            "description": v.description,
+            "like_count": v.like_count,
+            "in_db": True,
+            "channel_id": v.channel_id,
+            "channel_title": v.channel.title if v.channel else None,
+            "published_at": v.published_at.isoformat() if v.published_at else None,
+            "view_count": v.view_count,
+            "duration_seconds": v.duration_seconds,
+        }
+
+    try:
+        with quota.attribute(user.id, "video_lookup"), YouTubeClient(db, user) as yt:
+            items = yt.get_videos([video_id])
+    except YouTubeError as exc:
+        raise HTTPException(status_code=502, detail=f"YouTube lookup failed: {exc}")
+    if not items:
         raise HTTPException(status_code=404, detail="Unknown video")
+
+    snippet = items[0].get("snippet", {})
+    stats = items[0].get("statistics", {})
+    likes = stats.get("likeCount")
+    views = stats.get("viewCount")
     return {
-        "id": v.id,
-        "description": v.description,
-        "like_count": v.like_count,
+        "id": video_id,
+        "description": snippet.get("description"),
+        "like_count": int(likes) if likes is not None else None,
+        "in_db": False,
+        "channel_id": snippet.get("channelId"),
+        "channel_title": snippet.get("channelTitle"),
+        "published_at": snippet.get("publishedAt"),
+        "view_count": int(views) if views is not None else None,
+        "duration_seconds": parse_iso8601_duration(
+            items[0].get("contentDetails", {}).get("duration")
+        ),
     }