Fix mobile black-screen: declared video dimensions didn't match the file

ProcessedMedia.width/height for downloaded videos were always VK's original
source dimensions (e.g. 3840x2160 for a 4K upload), passed straight through
to Telegram's sendVideo width/height params - even though the actual file
sent is capped to video_max_height (e.g. 720p) by _select_format. A client
declaring 4K but receiving a 720p stream is a known cause of black-screen
playback on strict mobile decoders, which can allocate the render surface
from the declared size before the real stream is probed - matches the
reported symptom exactly (fine on desktop and in PiP, black in Telegram
mobile's fullscreen decoder path).

Neither the faststart remux nor the negative-DTS fix addressed this, since
both were about the file's own internal structure, not the metadata
describing it externally.

Adds _probe_video_dims (ffprobe on the actual downloaded/remuxed file) and
uses its real width/height/duration instead of VK's pre-download values.
Verified on the actual failing video: now reports the real 1280x720 instead
of VK's original 3840x2160.

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

src/media_processor.py

@@ -207,6 +207,38 @@ class MediaProcessor:
         await unregister_active_path(path)
         return fixed_path
 
+    async def _probe_video_dims(self, path: Path) -> tuple[Optional[int], Optional[int], Optional[float]]:
+        """Reads the ACTUAL encoded width/height/duration of a downloaded file via
+        ffprobe. Telegram was declared VK's original video dimensions (e.g. a 4K
+        source) while the file we actually send is capped to video_max_height (e.g.
+        720p) - that mismatch between declared and real dimensions is a known cause of
+        black-screen playback on strict mobile decoders, since some clients allocate
+        the render surface from the declared size before the real stream is probed."""
+        try:
+            proc = await asyncio.create_subprocess_exec(
+                "ffprobe", "-v", "error", "-select_streams", "v:0",
+                "-show_entries", "stream=width,height:format=duration",
+                "-of", "csv=p=0",
+                str(path),
+                stdout=asyncio.subprocess.PIPE, stderr=asyncio.subprocess.DEVNULL,
+            )
+            out, _ = await proc.communicate()
+        except OSError:
+            return None, None, None
+        lines = [l for l in out.decode().strip().splitlines() if l]
+        width = height = None
+        duration = None
+        for line in lines:
+            parts = line.split(",")
+            if len(parts) == 2 and parts[0].isdigit() and parts[1].isdigit():
+                width, height = int(parts[0]), int(parts[1])
+            elif len(parts) == 1:
+                try:
+                    duration = float(parts[0])
+                except ValueError:
+                    pass
+        return width, height, duration
+
     async def _download_video_ytdlp_attempt(
         self, url: str, prefix: str = "video_"
     ) -> tuple[Optional[Path], Optional[str], bool]:
@@ -373,6 +405,7 @@ class MediaProcessor:
 
                     p, err, perm = await self.download_video_ytdlp(url, prefix=f"v_{att_id}_")
                     if p:
+                        real_width, real_height, real_duration = await self._probe_video_dims(p)
                         results.append(
                             ProcessedMedia(
                                 media_type="video",
@@ -380,9 +413,9 @@ class MediaProcessor:
                                 original_url=url,
                                 attachment_id=att_id,
                                 size_bytes=p.stat().st_size,
-                                duration_sec=item.duration_sec,
-                                width=item.width,
-                                height=item.height,
+                                duration_sec=int(real_duration) if real_duration else item.duration_sec,
+                                width=real_width or item.width,
+                                height=real_height or item.height,
                             )
                         )
                     else: