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

test_get_voters.py

@@ -13,6 +13,7 @@ from datetime import datetime, timedelta
 
 import requests
 
+import db
 import get_voters as gv
 
 
@@ -69,7 +70,7 @@ def check(tag, cond):
 
 def main():
     tmp = tempfile.mkdtemp()
-    gv.DB_PATH = os.path.join(tmp, 'test.db')
+    db.DB_PATH = os.path.join(tmp, 'test.db')
 
     # V1: ulang ≥3x saat node timeout → hits, lalu sukses
     calls = {'n': 0}
@@ -180,7 +181,7 @@ def main():
         {'owner': 'ccc3', 'weight': '222.0', 'staked': 1000.0},
     ]
     gv.persist(set_a)
-    conn = sqlite3.connect(gv.DB_PATH)
+    conn = sqlite3.connect(db.DB_PATH)
     cnt = conn.execute('SELECT COUNT(*) FROM voters').fetchone()[0]
     conn.close()
     check('V7 snapshot pertama = 2', cnt == 2)
@@ -190,7 +191,7 @@ def main():
         {'owner': 'ddd4', 'weight': '333.0', 'staked': 2000.0},
     ]
     gv.persist(set_b)
-    conn = sqlite3.connect(gv.DB_PATH)
+    conn = sqlite3.connect(db.DB_PATH)
     rows = conn.execute('SELECT owner, weight, staked, scanned_at FROM voters').fetchall()
     by = {r[0]: r for r in rows}
     cols = [r[1] for r in conn.execute('PRAGMA table_info(voters)').fetchall()]