fix(v2): Handle bytes expected_ack from meshcore + DM prefix matching

- Convert bytes to hex string for expected_ack and pkt_payload via _to_str()
- Support pubkey prefix matching in get_dm_messages() (LIKE for short keys)
- Fixes "Object of type bytes is not JSON serializable" error on DM view

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/database.py

@@ -291,13 +291,20 @@ class Database:
                          offset: int = 0) -> List[Dict]:
         contact_pubkey = contact_pubkey.lower()
         with self._connect() as conn:
+            # Support both full key and prefix matching
+            if len(contact_pubkey) < 64:
+                condition = "contact_pubkey LIKE ?"
+                param = contact_pubkey + '%'
+            else:
+                condition = "contact_pubkey = ?"
+                param = contact_pubkey
             rows = conn.execute(
-                """SELECT * FROM (
+                f"""SELECT * FROM (
                     SELECT * FROM direct_messages
-                    WHERE contact_pubkey = ?
+                    WHERE {condition}
                     ORDER BY timestamp DESC LIMIT ? OFFSET ?
                 ) ORDER BY timestamp ASC""",
-                (contact_pubkey, limit, offset)
+                (param, limit, offset)
             ).fetchall()
             return [dict(r) for r in rows]