* fix(gateway): validate PID ownership and clean stale pid files
- include `pid` in health responses for runtime PID verification
- add `RemovePidFileIfPID` to safely delete PID files only on PID match
- sanitize gateway PID data via process-command checks with health fallback
- ignore and remove stale/non-gateway PID files before gateway operations
- refuse stop/restart actions when the attached process is not a gateway
- update gateway and websocket tests to cover PID validation and safety paths
* test(seahorse): use shared in-memory SQLite DB in tests to fix async compaction failures
* test: remove unused sendMediaErr field from hook test mock
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