web: implement a 'protocol' field across systems (#655)

* 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

data/traces.sql

@@ -22,7 +22,8 @@ CREATE TABLE IF NOT EXISTS traces (
     rssi        INTEGER,
     snr         REAL,
     elapsed_ms  INTEGER,
-    ingestor    TEXT
+    ingestor    TEXT,
+    protocol    TEXT NOT NULL DEFAULT 'meshtastic'
 );
 
 CREATE TABLE IF NOT EXISTS trace_hops (