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/api/avatar.py

@@ -17,7 +17,7 @@ from fastapi import APIRouter, Query, Response
 from app.accounts.ssrf import SsrfError, guarded_request
 from app.config import get_settings
 from app.identity.deps import CurrentUser
-from app.redis_client import get_redis
+from app.redis_client import get_redis_bytes
 
 router = APIRouter(prefix="/avatar", tags=["mail"])
 
@@ -66,7 +66,8 @@ async def sender_avatar(_user: CurrentUser, email: str = Query(...)) -> Response
     if not get_settings().sender_avatars_enabled or "@" not in email:
         return Response(status_code=404)
 
-    redis = get_redis()
+    # Binary-safe client: the cache holds raw image bytes (and a b"none" marker).
+    redis = get_redis_bytes()
     md5 = _email_md5(email)
     key = f"avatar:{md5}"
     headers = {"Cache-Control": "private, max-age=86400"}

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