refactor: extract TableMigrations and DbStore base class

Adds bot_bottle/migrations.py (TableMigrations) and bot_bottle/db_store.py
(DbStore) per PR review. Both stores now inherit from DbStore and hold a
TableMigrations instance instead of duplicating schema-version logic inline.

bot_bottle/db_store.py

@@ -0,0 +1,40 @@
+"""Shared SQLite-backed store base class for bot-bottle (PRD 0013)."""
+
+from __future__ import annotations
+
+import sqlite3
+from pathlib import Path
+
+try:
+    from .migrations import TableMigrations
+except ImportError:
+    from migrations import TableMigrations  # type: ignore[import-not-found]  # pylint: disable=import-error,no-name-in-module
+
+
+class DbStore:
+    """Base for SQLite-backed stores. Subclasses resolve db_path then call super().__init__."""
+
+    def __init__(self, db_path: Path, migrations: TableMigrations) -> None:
+        self.db_path = db_path
+        self._migrations = migrations
+        self.db_path.parent.mkdir(parents=True, exist_ok=True)
+        self._init()
+
+    def _connect(self) -> sqlite3.Connection:
+        conn = sqlite3.connect(self.db_path)
+        conn.row_factory = sqlite3.Row
+        return conn
+
+    def _init(self) -> None:
+        with self._connect() as conn:
+            self._migrations.apply(conn)
+        self._chmod()
+
+    def _chmod(self) -> None:
+        try:
+            self.db_path.chmod(0o600)
+        except OSError:
+            pass
+
+
+__all__ = ["DbStore"]