chore: move backend to separate dir

.gitignore

@@ -12,3 +12,6 @@ wheels/
 # Local application data
 log/
 *.db
+
+# Gradle
+local.properties

backend/init-db.py

@@ -16,6 +16,7 @@ cur.execute("CREATE TABLE devices(access_key, secret_key_hash, type, name)")
 # type                : INCOMING, OUTGOING
 # local_phone_number  : a SECONDARY device SIM's phone number
 # remote_phone_number : a phone number or shortcode of the other party
+# TODO add column for sender's access key
 cur.execute("CREATE TABLE messages(content, ts_received, ts_sent, type, local_phone_number, remote_phone_number)")
 cur.execute("CREATE TABLE sim_events(sim_id, ts, note, cost, currency)")
 cur.execute("CREATE TABLE sim_cards(phone_number, device_access_key)")