fix: wrong error variable in copy log, remove debug fmt.Println calls

[fix2015]

Two issues:

• `datacopy.go` line 273 was logging `err` instead of `cpErr` when a document copy failed. At that point `err` was nil from a previous successful call, so every copy failure logged `err: ` making it impossible to debug copy issues in production.

• `loop.go` and `batch.go` had `fmt.Println` debug statements left in production code. These bypass the structured logging system (`logs.CtxInfof`), can't be filtered by log level, and leak internal state info to stdout.

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