fix(channel/signal): route UUID senders as direct recipients

Privacy-enabled Signal users have no sourceNumber, so sender()
falls back to their UUID from the source field.  Previously
parse_recipient_target() treated non-E.164 strings without the
group: prefix as group IDs, causing signal-cli to reject the
UUID as an invalid base64 group ID.

Add is_uuid() helper using the already-imported uuid crate and
recognise valid UUIDs as Direct targets alongside E.164 numbers.

src/channels/signal.rs

@@ -119,12 +119,18 @@ impl SignalChannel {
         (2..=15).contains(&number.len()) && number.chars().all(|c| c.is_ascii_digit())
     }
 
+    /// Check whether a string is a valid UUID (signal-cli uses these for
+    /// privacy-enabled users who have opted out of sharing their phone number).
+    fn is_uuid(s: &str) -> bool {
+        Uuid::parse_str(s).is_ok()
+    }
+
     fn parse_recipient_target(recipient: &str) -> RecipientTarget {
         if let Some(group_id) = recipient.strip_prefix(GROUP_TARGET_PREFIX) {
             return RecipientTarget::Group(group_id.to_string());
         }
 
-        if Self::is_e164(recipient) {
+        if Self::is_e164(recipient) || Self::is_uuid(recipient) {
             RecipientTarget::Direct(recipient.to_string())
         } else {
             RecipientTarget::Group(recipient.to_string())
@@ -653,6 +659,15 @@ mod tests {
         );
     }
 
+    #[test]
+    fn parse_recipient_target_uuid_is_direct() {
+        let uuid = "a1b2c3d4-e5f6-7890-abcd-ef1234567890";
+        assert_eq!(
+            SignalChannel::parse_recipient_target(uuid),
+            RecipientTarget::Direct(uuid.to_string())
+        );
+    }
+
     #[test]
     fn parse_recipient_target_non_e164_plus_is_group() {
         assert_eq!(
@@ -661,6 +676,24 @@ mod tests {
         );
     }
 
+    #[test]
+    fn is_uuid_valid() {
+        assert!(SignalChannel::is_uuid(
+            "a1b2c3d4-e5f6-7890-abcd-ef1234567890"
+        ));
+        assert!(SignalChannel::is_uuid(
+            "00000000-0000-0000-0000-000000000000"
+        ));
+    }
+
+    #[test]
+    fn is_uuid_invalid() {
+        assert!(!SignalChannel::is_uuid("+1234567890"));
+        assert!(!SignalChannel::is_uuid("not-a-uuid"));
+        assert!(!SignalChannel::is_uuid("group:abc123"));
+        assert!(!SignalChannel::is_uuid(""));
+    }
+
     #[test]
     fn sender_prefers_source_number() {
         let env = Envelope {
@@ -685,6 +718,73 @@ mod tests {
         assert_eq!(SignalChannel::sender(&env), Some("uuid-123".to_string()));
     }
 
+    #[test]
+    fn process_envelope_uuid_sender_dm() {
+        let uuid = "a1b2c3d4-e5f6-7890-abcd-ef1234567890";
+        let ch = SignalChannel::new(
+            "http://127.0.0.1:8686".to_string(),
+            "+1234567890".to_string(),
+            None,
+            vec!["*".to_string()],
+            false,
+            false,
+        );
+        let env = Envelope {
+            source: Some(uuid.to_string()),
+            source_number: None,
+            data_message: Some(DataMessage {
+                message: Some("Hello from privacy user".to_string()),
+                timestamp: Some(1_700_000_000_000),
+                group_info: None,
+                attachments: None,
+            }),
+            story_message: None,
+            timestamp: Some(1_700_000_000_000),
+        };
+        let msg = ch.process_envelope(&env).unwrap();
+        assert_eq!(msg.sender, uuid);
+        assert_eq!(msg.reply_target, uuid);
+        assert_eq!(msg.content, "Hello from privacy user");
+
+        // Verify reply routing: UUID sender in DM should route as Direct
+        let target = SignalChannel::parse_recipient_target(&msg.reply_target);
+        assert_eq!(target, RecipientTarget::Direct(uuid.to_string()));
+    }
+
+    #[test]
+    fn process_envelope_uuid_sender_in_group() {
+        let uuid = "a1b2c3d4-e5f6-7890-abcd-ef1234567890";
+        let ch = SignalChannel::new(
+            "http://127.0.0.1:8686".to_string(),
+            "+1234567890".to_string(),
+            Some("testgroup".to_string()),
+            vec!["*".to_string()],
+            false,
+            false,
+        );
+        let env = Envelope {
+            source: Some(uuid.to_string()),
+            source_number: None,
+            data_message: Some(DataMessage {
+                message: Some("Group msg from privacy user".to_string()),
+                timestamp: Some(1_700_000_000_000),
+                group_info: Some(GroupInfo {
+                    group_id: Some("testgroup".to_string()),
+                }),
+                attachments: None,
+            }),
+            story_message: None,
+            timestamp: Some(1_700_000_000_000),
+        };
+        let msg = ch.process_envelope(&env).unwrap();
+        assert_eq!(msg.sender, uuid);
+        assert_eq!(msg.reply_target, "group:testgroup");
+
+        // Verify reply routing: group message should still route as Group
+        let target = SignalChannel::parse_recipient_target(&msg.reply_target);
+        assert_eq!(target, RecipientTarget::Group("testgroup".to_string()));
+    }
+
     #[test]
     fn sender_none_when_both_missing() {
         let env = Envelope {