refactor: use public key prefix for DB filename instead of device name

DB filename changes from {device_name}.db to mc_{pubkey[:8]}.db,
making it stable across device renames and preparing for multi-device support.
Existing databases are auto-migrated at startup by probing the device table.

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

app/database.py

@@ -80,6 +80,12 @@ class Database:
             row = conn.execute("SELECT * FROM device WHERE id = 1").fetchone()
             return dict(row) if row else None
 
+    def get_public_key(self) -> Optional[str]:
+        """Get device public key (used for DB filename resolution)."""
+        with self._connect() as conn:
+            row = conn.execute("SELECT public_key FROM device WHERE id = 1").fetchone()
+            return row['public_key'] if row and row['public_key'] else None
+
     # ================================================================
     # Contacts
     # ================================================================