fix: prevent memory/CPU leaks in importer and nzbfilesystem

[javi11]

Summary

• Replace fire-and-forget goroutines in updateFileHealthOnError with synchronous operations using a 5-second timeout context to prevent unbounded goroutine accumulation when reading corrupted files
• Wrap segment reader operations in parser.go with closure and defer to ensure cleanup happens regardless of how the loop exits
• Add channel draining in NzbDav import defer block to prevent parser goroutine from blocking indefinitely when import is cancelled

Test plan

• Verify corrupted file health updates still work correctly
• Test NzbDav import with cancellation
• Monitor goroutine count under load using pprof or runtime.NumGoroutine()

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