* web: implement a 'protocol' field across systems
* web: address review feedback on multi-protocol support
- Rebase on main (pick up coordinate-clearing bugfix from #654)
- P1: prevent cross-protocol message merges on shared packet IDs
- P2: exclude "ingestor" key when enforcing /api/nodes batch limit
- Extract append_protocol_filter helper + PROTOCOL_CLAUSE constant to
reduce cognitive complexity and deduplicate SQL fragment in queries.rb
- Extract coerce_bool helper to reduce upsert_node cognitive complexity
- Merge nested if in insert_message protocol update path (Sonar)
- Add explicit UPDATE backfill in ensure_schema_upgrades so any pre-existing
NULL/empty protocol rows are set to meshtastic on upgrade
- Rename migration file to 20260328_ (correct year)
- Expand protocol_spec.rb: filter tests for all 7 endpoints,
cross-protocol non-merge test, batch limit test, Sonar constant fixes,
ENV.fetch, P1 regression test
* web: address review comments
