Retry transient photo download failures instead of failing permanently

Photo downloads had zero retries, unlike video (which already retries
stalls/timeouts). Confirmed live: a photo whose download hit "500" from
VK's own CDN (sun9-87.userapi.com) was back to a normal 200 moments later -
a one-off hiccup, not a real problem with the photo - but with no retry, it
went straight to the link-only fallback for good, since a published post is
never revisited. Now retries up to 2 more times (3s apart) on a 5xx status,
timeout, or connection error; a 4xx (permanently gone/bad URL) still fails
immediately since retrying that wouldn't help.

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

src/media_processor.py

@@ -70,9 +70,34 @@ class MediaProcessor:
     async def download_photo(
         self, session: aiohttp.ClientSession, url: str, prefix: str = "photo_"
     ) -> Optional[Path]:
+        """Retries a couple of times on a transient failure (5xx, timeout, connection
+        reset) before giving up - unlike video, this used to have zero retries, so a
+        single one-off hiccup on VK's CDN (confirmed live: a photo that 500'd here was
+        back to 200 OK moments later) sent a perfectly fine photo straight to the
+        link-only fallback, forever, since a published post never gets revisited. A 4xx
+        (bad URL, permanently gone) is not worth retrying and fails immediately."""
         if not url.startswith(("http://", "https://")):
             return None
 
+        attempts = 3
+        for attempt in range(1, attempts + 1):
+            path, status, exc = await self._download_photo_attempt(session, url, prefix)
+            if path:
+                return path
+            if status is not None and 400 <= status < 500:
+                logger.warning("Photo download failed with status {} (permanent): {}", status, url)
+                return None
+            if attempt < attempts:
+                logger.warning(
+                    "Photo download attempt {}/{} failed transiently ({}) for {} - retrying",
+                    attempt, attempts, status if status is not None else exc, url,
+                )
+                await asyncio.sleep(3.0)
+        return None
+
+    async def _download_photo_attempt(
+        self, session: aiohttp.ClientSession, url: str, prefix: str
+    ) -> tuple[Optional[Path], Optional[int], Optional[Exception]]:
         suffix = Path(url.split("?", 1)[0]).suffix[:6] or ".jpg"
         tmp = tempfile.NamedTemporaryFile(
             dir=self.cache_dir, prefix=prefix, suffix=suffix, delete=False
@@ -86,19 +111,17 @@ class MediaProcessor:
                 url, timeout=aiohttp.ClientTimeout(total=settings.media_download_timeout_sec)
             ) as resp:
                 if resp.status != 200:
-                    logger.warning("Photo download failed with status {}: {}", resp.status, url)
                     tmp_path.unlink(missing_ok=True)
                     await unregister_active_path(tmp_path)
-                    return None
+                    return None, resp.status, None
                 with open(tmp_path, "wb") as f:
                     async for chunk in resp.content.iter_chunked(64 * 1024):
                         f.write(chunk)
-            return tmp_path
+            return tmp_path, None, None
         except Exception as exc:
-            logger.warning("Error downloading photo {}: {}", url, exc)
             tmp_path.unlink(missing_ok=True)
             await unregister_active_path(tmp_path)
-            return None
+            return None, None, exc
 
     async def download_video_ytdlp(
         self, url: str, prefix: str = "video_"