T17: backend penyimpanan bersama db.py (sqlite | mysql via PyMySQL)

- db.py: abstraksi connect/query/queryone/replace_snapshot; %s → ? utk sqlite;
  replace snapshot mysql = CREATE IF NOT EXISTS + DELETE + INSERT (MVCC)
- config/.env.example: VEX_DB_BACKEND (default sqlite) + VEX_DB_HOST/PORT/USER/PASS/NAME
- get_voters.persist → db.replace_snapshot; dashboard baca via db.query
- LIKE pakai ESCAPE '!' (backslash memutus literal string MySQL, B2);
  db.query selalu kembalikan list (PyMySQL tuple vs sqlite list, B3)
- docker-compose.yml: kontainer uji mariadb:11 (databisnisid-mariadb, 127.0.0.1:3306)
- test_mariadb.py: suite integrasi opt-in (skip bila bukan mysql)
- SPEC.md §V17/T17/B2/B3; AGENTS.md runbook

config.py

@@ -19,6 +19,16 @@ API_NODE = os.getenv('VEX_API_NODE', 'https://v2.vexascan.com:2096')
 # Lokasi basis data SQLite (dipakai fetcher & dashboard)
 DB_PATH = os.getenv('VEX_DB_PATH', 'voters.db')
 
+# Backend basis data: 'sqlite' (default) atau 'mysql' (MariaDB/MySQL via PyMySQL)
+DB_BACKEND = os.getenv('VEX_DB_BACKEND', 'sqlite')
+
+# Koneksi MariaDB/MySQL (dipakai saat DB_BACKEND='mysql')
+DB_HOST = os.getenv('VEX_DB_HOST', '127.0.0.1')
+DB_PORT = int(os.getenv('VEX_DB_PORT', '3306'))
+DB_USER = os.getenv('VEX_DB_USER', '')
+DB_PASS = os.getenv('VEX_DB_PASS', '')
+DB_NAME = os.getenv('VEX_DB_NAME', '')
+
 # Syarat minimum stake (dalam VEX) untuk disimpan
 MIN_STAKED_VEX = float(os.getenv('VEX_MIN_STAKED_VEX', '1000'))