Add TODO.md with remaining performance improvements

TODO.md

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+# Performance TODO
+
+## N+1 query in load_rf_path_station_packets
+
+`index.ex:895` — `handle_info({:load_rf_path_station_packets, stations})` still does N+1
+`Packets.get_latest_packet_for_callsign/1` calls per station. Unlike the RF path position
+lookup (which only needs lat/lng), this needs full packet data for building markers with
+popups. A batch version of `get_latest_packet_for_callsign` returning full packets via
+`DISTINCT ON` would eliminate this.
+
+## Batch remove_marker events
+
+`PacketProcessor.batch_dispatch({false, true}, ...)` still pushes individual `remove_marker`
+events during batch processing. These are rare (packet goes out of bounds) but could be
+batched into a single `remove_markers` event with a list of IDs if profiling shows it matters.
+
+## PostgreSQL table partitioning
+
+The packets table uses CTE-based batch DELETE for cleanup (7-day retention). Time-range
+partitioning would make cleanup instant (DROP partition) and improve query performance for
+time-bounded queries. This is a significant migration effort.
+
+## ETS table for :aprsme — write path
+
+Changed `:aprsme` from `write_concurrency` to `read_concurrency`. The only write is
+`:message_number` increment. If message throughput increases significantly, consider using
+`:atomics` or `:counters` instead of ETS for the message counter.