Retry transient video download failures, friendlier fallback link text, enable previews

- yt-dlp: retry transient failures (stall/timeout) up to 2x before falling
  back to a link-only post; permanent errors (private/removed) still fail fast
- Fallback note: friendlier wording, "Смотреть видео" as link text instead of
  a bare "ссылка", plus a link to the original VK wall post as a backup
- TG: enable link preview for text-only fallback posts (was unconditionally
  disabled, leaving fallback posts with no visual at all)
- MAX: attach the VK video's own thumbnail as an image so failed/oversized
  video fallbacks still show a preview picture (MAX has no OG-preview for
  arbitrary URLs, but does accept a remote image url without upload)

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

src/media_processor.py

@@ -49,6 +49,7 @@ class ProcessedMedia:
     height: Optional[int] = None
     error: Optional[str] = None
     is_link_only: bool = False
+    thumbnail_url: Optional[str] = None
 
 
 class MediaProcessor:
@@ -100,6 +101,26 @@ class MediaProcessor:
 
     async def download_video_ytdlp(
         self, url: str, prefix: str = "video_"
+    ) -> tuple[Optional[Path], Optional[str], bool]:
+        """Retries transient failures (stalls, timeouts, one-off yt-dlp/network hiccups)
+        a couple of times before giving up - a single flaky attempt used to fall straight
+        through to the link-only fallback even though the video was perfectly downloadable."""
+        last_result: tuple[Optional[Path], Optional[str], bool] = (None, "video download failed", False)
+        for attempt in range(1, settings.yt_dlp_retry_attempts + 2):
+            path, err, is_permanent = await self._download_video_ytdlp_attempt(url, prefix)
+            if path or is_permanent:
+                return path, err, is_permanent
+            last_result = (path, err, is_permanent)
+            if attempt <= settings.yt_dlp_retry_attempts:
+                logger.warning(
+                    "yt-dlp attempt {}/{} failed transiently for {}: {} - retrying",
+                    attempt, settings.yt_dlp_retry_attempts + 1, url, err,
+                )
+                await asyncio.sleep(3.0)
+        return last_result
+
+    async def _download_video_ytdlp_attempt(
+        self, url: str, prefix: str = "video_"
     ) -> tuple[Optional[Path], Optional[str], bool]:
         """Returns (path, error_message, is_permanent_error)"""
         output_path = self.cache_dir / f"{prefix}{uuid.uuid4().hex[:12]}.mp4"
@@ -249,6 +270,7 @@ class MediaProcessor:
                                 duration_sec=item.duration_sec,
                                 error="video duration exceeds maximum",
                                 is_link_only=True,
+                                thumbnail_url=item.thumbnail_url,
                             )
                         )
                         continue
@@ -277,6 +299,7 @@ class MediaProcessor:
                                 duration_sec=item.duration_sec,
                                 error=err or "failed to download video",
                                 is_link_only=True,
+                                thumbnail_url=item.thumbnail_url,
                             )
                         )
         return results