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/messages.sql

@@ -30,7 +30,8 @@ CREATE TABLE IF NOT EXISTS messages (
     channel_name TEXT,
     reply_id INTEGER,
     emoji TEXT,
-    ingestor TEXT
+    ingestor TEXT,
+    protocol TEXT NOT NULL DEFAULT 'meshtastic'
 );
 
 CREATE INDEX IF NOT EXISTS idx_messages_rx_time   ON messages(rx_time);