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.
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