feat(channels): capture raw_packet at send time for raw resend

PR #2 of 5. Builds the full GRP_TXT wire bytes (header + transport_codes if
scoped + path_len + encrypted payload) from the ts+0 pkt_payload guess and
stores it in channel_messages.raw_packet right after the send. When echo
correlation later identifies the actual pkt_payload (potentially using a
different ±dt candidate due to host/firmware clock drift), the raw_packet is
rebuilt from the actual one so a future resend matches the original packet
hash and dedupes at the repeaters.

Transport-scope codes are computed in Python via HMAC-SHA256(scope_key,
payload_type||payload)[:2], mirroring TransportKey::calcTransportCode in
MeshCore Core (including the 0x0000/0xFFFF reservations).

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

app/database.py

@@ -993,6 +993,14 @@ class Database:
                 (pkt_payload, msg_id)
             )
 
+    def update_message_raw_packet(self, msg_id: int, raw_packet: str) -> None:
+        """Set raw_packet on a channel message (full wire bytes for raw resend)."""
+        with self._connect() as conn:
+            conn.execute(
+                "UPDATE channel_messages SET raw_packet = ? WHERE id = ?",
+                (raw_packet, msg_id)
+            )
+
     # ================================================================
     # Paths
     # ================================================================