feat(db_store): decouple migration from store construction

Remove auto-migration from DbStore.__init__; add explicit check_migrations()
and migrate() methods. The CLI now checks both stores on every startup and
prompts the user to confirm before migrating. supervise.prepare() calls
.migrate() directly now that __init__ no longer does it implicitly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot_bottle/supervise.py

@@ -357,8 +357,8 @@ class Supervise(ABC):
         must be set by the launch step before .start runs."""
         del stage_dir
         db_path = host_db_path()
-        QueueStore(slug)
-        AuditStore(db_path)
+        QueueStore(slug).migrate()
+        AuditStore(db_path).migrate()
         return SupervisePlan(
             slug=slug,
             db_path=db_path,