Received-file deduplication set grows without bound

[offbyonebit]

Severity: medium
Type: robustness
Location: clipsync/file_transfer.py — _FileReceiveHandler._handle() (~line 120)

Problem: _seen stores every received file path forever. Over a long-running instance this leaks memory, and it also suppresses legitimate later deliveries of a file with the same name from the same sender.

Suggested fix: Replace the unbounded set with a time-bounded cache (entries expire after a few seconds) or key on (path, mtime) so repeated intentional transfers are not dropped.

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_{Filed from a full-source audit of the repo (2026-07-29). Reported by the audit but not independently confirmed - verify the cited code before acting on it.}