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/migrations.py

@@ -0,0 +1,37 @@
+"""SQLite migration runner for bot-bottle stores."""
+
+from __future__ import annotations
+
+import sqlite3
+
+
+class TableMigrations:
+    """Runs a sequential list of DDL migrations tracked by schema_key in schema_versions."""
+
+    def __init__(self, schema_key: str, migrations: list[str]) -> None:
+        self.schema_key = schema_key
+        self.migrations = migrations
+
+    def apply(self, conn: sqlite3.Connection) -> None:
+        conn.execute(
+            """
+            CREATE TABLE IF NOT EXISTS schema_versions (
+                module TEXT PRIMARY KEY,
+                version INTEGER NOT NULL DEFAULT 0
+            )
+            """
+        )
+        row = conn.execute(
+            "SELECT version FROM schema_versions WHERE module = ?",
+            (self.schema_key,),
+        ).fetchone()
+        version = row[0] if row else 0
+        for i, sql in enumerate(self.migrations[version:], start=version + 1):
+            conn.execute(sql)
+            conn.execute(
+                "INSERT OR REPLACE INTO schema_versions (module, version) VALUES (?, ?)",
+                (self.schema_key, i),
+            )
+
+
+__all__ = ["TableMigrations"]