fix(dm): resolve short pubkey prefix to full key on incoming DM

When a DM arrives with only pubkey_prefix (short hex) and the sender
is not in mc.contacts, fall back to DB prefix lookup to get the full
64-char public key. Prevents ghost contact entries and "Unknown" DM
conversations.

Also adds get_contact_by_prefix() database helper.

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

app/database.py

@@ -128,6 +128,15 @@ class Database:
             ).fetchone()
             return dict(row) if row else None
 
+    def get_contact_by_prefix(self, prefix: str) -> Optional[Dict]:
+        """Find a contact by public key prefix (LIKE match)."""
+        with self._connect() as conn:
+            row = conn.execute(
+                "SELECT * FROM contacts WHERE public_key LIKE ? AND length(public_key) = 64 LIMIT 1",
+                (prefix.lower() + '%',)
+            ).fetchone()
+            return dict(row) if row else None
+
     def delete_contact(self, public_key: str) -> bool:
         with self._connect() as conn:
             cursor = conn.execute(

app/device_manager.py

@@ -389,10 +389,15 @@ class DeviceManager:
                 contact = self.mc.get_contact_by_key_prefix(sender_key)
                 if contact:
                     sender_name = contact.get('name', '')
-                    # Use the full public key from contacts (not the short prefix)
                     full_key = contact.get('public_key', '')
                     if full_key:
                         sender_key = full_key
+                elif len(sender_key) < 64:
+                    # Prefix not resolved from in-memory contacts — try DB
+                    db_contact = self.db.get_contact_by_prefix(sender_key)
+                    if db_contact and len(db_contact['public_key']) == 64:
+                        sender_key = db_contact['public_key']
+                        sender_name = db_contact.get('name', '')
 
             # Receiver-side dedup: skip duplicate retries
             sender_ts = data.get('sender_timestamp')