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

db.py

@@ -0,0 +1,97 @@
+"""Abstraksi penyimpanan: sqlite (default) atau MariaDB/MySQL (PyMySQL).
+
+Pilih backend via config `VEX_DB_BACKEND` (env `.env`). Kedua backend memakai
+skema `voters` yang sama; query berbagi sintaks ditulis dengan placeholder
+`%s` yang diterjemahkan ke `?` untuk sqlite. Dashboard read-only (V10).
+"""
+
+import sqlite3
+
+from config import (DB_BACKEND, DB_HOST, DB_NAME, DB_PASS, DB_PORT, DB_PATH,
+                    DB_USER)
+
+# Skema voters — sama untuk kedua backend (I.db)
+_COLS = (
+    'owner VARCHAR(13) PRIMARY KEY, '
+    'weight TEXT NOT NULL, '
+    'staked DOUBLE NOT NULL, '
+    'scanned_at TEXT NOT NULL, '
+    'last_vote TEXT'
+)
+_SCHEMA = f'CREATE TABLE voters ({_COLS})'
+_SCHEMA_IF_NOT_EXISTS = f'CREATE TABLE IF NOT EXISTS voters ({_COLS})'
+
+
+def _translate(sql):
+    """sqlite pakai `?`; mysql pakai `%s`. Ubah `%s` → `?` untuk sqlite."""
+    return sql.replace('%s', '?') if DB_BACKEND == 'sqlite' else sql
+
+
+def connect():
+    """Buka koneksi backend. sqlite: WAL agar pembaca tak terblokir."""
+    if DB_BACKEND == 'sqlite':
+        conn = sqlite3.connect(DB_PATH)
+        conn.execute('PRAGMA journal_mode=WAL')
+        return conn
+    if DB_BACKEND == 'mysql':
+        import pymysql
+        return pymysql.connect(
+            host=DB_HOST, port=DB_PORT, user=DB_USER, password=DB_PASS,
+            database=DB_NAME, charset='utf8mb4',
+        )
+    raise ValueError(f'VEX_DB_BACKEND tak dikenal: {DB_BACKEND!r}')
+
+
+def query(sql, params=()):
+    """Jalankan SELECT → daftar baris (tuple), koneksi dibuka-tutup tiap panggil."""
+    conn = connect()
+    try:
+        cur = conn.cursor()
+        cur.execute(_translate(sql), params)
+        return list(cur.fetchall())  # sqlite→list, PyMySQL→tuple: seragamkan
+    finally:
+        conn.close()
+
+
+def queryone(sql, params=()):
+    """Jalankan SELECT → baris pertama (tuple) atau None."""
+    conn = connect()
+    try:
+        cur = conn.cursor()
+        cur.execute(_translate(sql), params)
+        return cur.fetchone()
+    finally:
+        conn.close()
+
+
+def replace_snapshot(rows, scanned_at):
+    """V7: ganti snapshot voters tiap skan (⊥ history/append).
+
+    sqlite: DROP+CREATE+INSERT (WAL melindungi pembaca).
+    mysql: CREATE IF NOT EXISTS + DELETE + INSERT dalam satu transaksi
+    → MVCC memberi pembaca snapshot konsisten (⊥ torn read saat ganti harian).
+    """
+    tuples = [(r['owner'], r['weight'], r['staked'], scanned_at, r.get('last_vote'))
+              for r in rows]
+    conn = connect()
+    try:
+        cur = conn.cursor()
+        if DB_BACKEND == 'sqlite':
+            cur.execute('DROP TABLE IF EXISTS voters')
+            cur.execute(_SCHEMA)
+            cur.executemany(
+                'INSERT INTO voters (owner, weight, staked, scanned_at, last_vote) '
+                'VALUES (?, ?, ?, ?, ?)',
+                tuples,
+            )
+        else:
+            cur.execute(_SCHEMA_IF_NOT_EXISTS)
+            cur.execute('DELETE FROM voters')
+            cur.executemany(
+                'INSERT INTO voters (owner, weight, staked, scanned_at, last_vote) '
+                'VALUES (%s, %s, %s, %s, %s)',
+                tuples,
+            )
+        conn.commit()
+    finally:
+        conn.close()