feat(db): add channel_messages.raw_packet column for raw resend

Stores the full hex packet snapshot (header+transport_codes+path_len+payload)
captured at send time, enabling future "Resend (same packet hash)" feature
that lets repeaters deduplicate via Packet::calculatePacketHash while still
forwarding to nodes that didn't hear the original.

NULL for received and pre-migration rows (resend button stays disabled there).
PR #1 of 5; subsequent PRs wire up send-time capture, backend endpoint, echo
list merge, and UI.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/schema.sql

@@ -86,6 +86,7 @@ CREATE TABLE IF NOT EXISTS channel_messages (
     snr                 REAL,
     path_len            INTEGER,
     pkt_payload         TEXT,                               -- for echo matching
+    raw_packet          TEXT,                               -- full hex packet (header+codes+path_len+payload) for raw resend; NULL for received/legacy rows
     raw_json            TEXT,                               -- original JSON line
     created_at          TEXT NOT NULL DEFAULT (datetime('now'))
 );