feat: integrate Alembic for database migrations

Replace manual if-sqlite/if-postgres branching with Alembic:
- Add alembic dependency
- Create programmatic alembic env (no CLI/alembic.ini needed)
- Support async engines via run_sync passthrough
- render_as_batch=True for SQLite ALTER TABLE compatibility
- Auto-stamp baseline on first run (existing DB at version 25)
- Run alembic upgrade head after legacy migrations
- Include sample migration showing schema + data migration patterns
- Add alembic dir to package-data for distribution

src/langbot/pkg/persistence/alembic/versions/0001_baseline.py

@@ -0,0 +1,24 @@
+"""baseline: stamp existing schema (db version 25)
+
+This is a no-op migration that marks the starting point for Alembic.
+All tables already exist via create_all() + legacy DBMigration system.
+
+Revision ID: 0001_baseline
+Revises: None
+Create Date: 2026-04-08
+"""
+
+revision = '0001_baseline'
+down_revision = None
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # No-op: existing schema is already at database_version=25
+    # This revision serves as the Alembic baseline.
+    pass
+
+
+def downgrade() -> None:
+    pass