Internally Managed Migrations (#438)

This PR replaces Alembic-based system database migrations with a simpler
internal system.

Migrations are now an array of SQL strings in `_migration.py`. There is
a new table `dbos.dbos_migrations` in the system database that keeps
track of which has run. When initializing the system database, it looks
up that table to see which migrations have already run and then runs the
rest.

This new scheme is fully backwards-compatible with the old
(Alembic-based) migrations, as this PR also adds a final Alembic
migration that creates the `dbos.dbos_migrations` table and inserts an
initial version into it.

We will move all DBOS languages (TypeScript, Go, Java) to this system
and use exactly the same migrations for each to ensure system database
compatibility across languages.

Author: kraftp

DEVELOPING.md

@@ -86,17 +86,6 @@ on the changes in the project since this was written):
 Success: no issues found in 64 source files
 ```
 
-We use alembic to manage system table schema migrations.
-To generate a new migration, run:
-
-```
-pdm run alembic revision -m "<new migration name>"
-```
-
-This command will add a new file under the `dbos/migrations/versions/` folder.
-For more information,
-read [alembic tutorial](https://alembic.sqlalchemy.org/en/latest/tutorial.html).
-
 ### Creating a Release
 
 To cut a new release, run:

alembic.ini

@@ -0,0 +1,35 @@
+"""dbos_migrations
+
+Revision ID: 471b60d64126
+Revises: 01ce9f07bd10
+Create Date: 2025-08-21 14:22:31.455266
+
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "471b60d64126"
+down_revision: Union[str, None] = "01ce9f07bd10"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Create dbos_migrations table
+    op.create_table(
+        "dbos_migrations",
+        sa.Column("version", sa.BigInteger(), nullable=False),
+        sa.PrimaryKeyConstraint("version"),
+        schema="dbos",
+    )
+
+    # Insert initial version 1
+    op.execute("INSERT INTO dbos.dbos_migrations (version) VALUES (1)")
+
+
+def downgrade() -> None:
+    op.drop_table("dbos_migrations", schema="dbos")