Avatars: a cached avatar or miss failed on every later request

The shared Redis client decodes replies as UTF-8 text, but the avatar cache
holds raw image bytes and a b"none" miss marker: an image could not be read
back at all, and a miss came back as str and crashed on .partition(b"\n").
Only a sender's first request worked; after that, a 500. The cache now goes
through a binary-safe client, get_redis_bytes().

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/redis_client.py

@@ -20,6 +20,7 @@ def aw(result: Any) -> Awaitable[Any]:
 
 
 _client: aioredis.Redis | None = None
+_bytes_client: aioredis.Redis | None = None
 
 
 def get_redis() -> aioredis.Redis:
@@ -34,9 +35,25 @@ def get_redis() -> aioredis.Redis:
     return _client
 
 
+def get_redis_bytes() -> aioredis.Redis:
+    """Return the process-wide Redis client for binary values.
+
+    ``get_redis()`` decodes every reply as UTF-8 text, which fails on (or
+    silently changes the type of) raw bytes such as a cached image. Anything that
+    stores bytes goes through this one instead.
+    """
+    global _bytes_client
+    if _bytes_client is None:
+        _bytes_client = aioredis.Redis.from_url(get_settings().redis_url, decode_responses=False)
+    return _bytes_client
+
+
 async def close_redis() -> None:
-    """Close the Redis client (used on app shutdown)."""
-    global _client
+    """Close the Redis clients (used on app shutdown)."""
+    global _client, _bytes_client
     if _client is not None:
         await _client.aclose()
         _client = None
+    if _bytes_client is not None:
+        await _bytes_client.aclose()
+        _bytes_client = None