fix(channels): reconcile an already-followed channel instead of erroring

Subscribing to a channel the user already follows on YouTube (a local/YouTube
desync — e.g. a stale import dropped the local row, so it resurfaced in the
discovery list) made YouTube return 400 "subscription already exists", which
the endpoint reported as a 502 — and the client shows a 502 as a transient
"connection lost" blip, not the real reason. Treat the duplicate as success:
record the subscription locally (the desired end state already holds; the next
resync fills in the resource id). Surface any other YouTube error as 422 with a
clear message, on both subscribe and unsubscribe, so it isn't mistaken for a
connection drop.

backend/app/routes/channels.py

@@ -313,7 +313,17 @@ def subscribe(
             if channel.details_synced_at is None:
                 apply_channel_details(db, yt.get_channels([channel_id]))
     except YouTubeError as exc:
-        raise HTTPException(status_code=502, detail=f"YouTube subscribe failed: {exc}")
+        # Already following on YouTube but not recorded here (a desync — e.g. a stale import
+        # dropped the local row, so it resurfaced in discovery). That's not a failure: the end
+        # state the user wanted already holds, so record it locally and move on. The next
+        # subscription resync fills in the resource id. Any other YouTube error is a real
+        # problem — surface it with a clear message (422, not a 502 that the client would
+        # mistake for a transient "connection lost").
+        msg = str(exc).lower()
+        if "already exists" in msg or "duplicate" in msg:
+            yt_sub_id = ""
+        else:
+            raise HTTPException(status_code=422, detail=f"YouTube couldn't subscribe you: {exc}")
     db.add(
         Subscription(
             user_id=user.id,
@@ -349,7 +359,9 @@ def unsubscribe(
         with quota.attribute(user.id, "unsubscribe"), YouTubeClient(db, user) as yt:
             yt.delete_subscription(sub.yt_subscription_id)
     except YouTubeError as exc:
-        raise HTTPException(status_code=502, detail=f"YouTube unsubscribe failed: {exc}")
+        # 422 (a real, explainable error → speaking modal), not 502 which the client treats as
+        # a transient "connection lost" gateway blip.
+        raise HTTPException(status_code=422, detail=f"YouTube couldn't unsubscribe you: {exc}")
     db.delete(sub)
     db.commit()
     return {"unsubscribed": channel_id}