WHAT’S NEW
• controller/ratio_sync.go
– Deleted unused local structs (TestResult, DifferenceItem, SyncableChannel).
– Centralised config with constants: defaultTimeoutSeconds, defaultEndpoint, maxConcurrentFetches, ratioTypes.
– Replaced magic numbers; added semaphore-based concurrency limit and shared http.Client (with TLS & Expect-Continue timeouts).
– Added comprehensive error handling and context-aware logging via common.Log* helpers.
– Checked DB errors from GetChannelsByIds; early-return on failures or empty upstream list.
– Removed custom-channel support; logic now relies solely on ChannelIDs.
– Minor clean-ups: import grouping, string trimming, endpoint normalisation.
• dto/ratio_sync.go
– Simplified UpstreamRequest: dropped unused CustomChannels field.
WHY
These improvements harden the ratio-sync endpoint for production use by preventing silent failures, controlling resource usage, and making behaviour configurable and observable.
HOW
No business logic change—only structural refactor, logging, and safeguards—so existing API contracts (aside from removed custom_channels) remain intact.
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