fix(udp): ignore N0CALL, and stop returning a grid as a callsign

N0CALL is what WSJT-X transmits under when its owner never set a callsign. It
has a letter, a digit and an ordinary shape, so nothing rejected it: it was
spotted, coloured, counted as a new WPX prefix, and now that CQ grids are read
it would have put a grid into the worked index under a callsign nobody holds.

The obvious fix - adding it to looksLikeCall's reject list - was wrong, and the
test caught it before it shipped. That function answers "could this token be a
callsign at all", and the CQ grammar uses it to decide whether the word after CQ
is a modifier (DX, NA, a zone) or the call itself. Teaching it that N0CALL is
not a callsign made "CQ N0CALL JN36" skip a slot and return JN36. Shape and
policy are different questions and now live in different functions.

Chasing that turned up the real defect behind it: ANY unrecognised word after CQ
made the parser skip a slot, and a four-character grid passes every shape test a
callsign does. "CQ FOO JN36" returned JN36 as the sender - logged, spotted and
coloured as a station. A grid in the callsign slot is now refused outright.

internal/integrations/udp/config.go

@@ -41,16 +41,16 @@ const (
 
 // Config is one user-defined UDP connection.
 type Config struct {
-	ID             int64     `json:"id"`
-	Direction      Direction `json:"direction"`
-	Name           string    `json:"name"`
-	Port           int       `json:"port"`
+	ID             int64       `json:"id"`
+	Direction      Direction   `json:"direction"`
+	Name           string      `json:"name"`
+	Port           int         `json:"port"`
 	ServiceType    ServiceType `json:"service_type"`
-	Multicast      bool      `json:"multicast"`
-	MulticastGroup string    `json:"multicast_group"`
-	DestinationIP  string    `json:"destination_ip"` // outbound only
-	Enabled        bool      `json:"enabled"`
-	SortOrder      int       `json:"sort_order"`
+	Multicast      bool        `json:"multicast"`
+	MulticastGroup string      `json:"multicast_group"`
+	DestinationIP  string      `json:"destination_ip"` // outbound only
+	Enabled        bool        `json:"enabled"`
+	SortOrder      int         `json:"sort_order"`
 }
 
 // Repo is the persistence layer for UDP integration rows.