build(db): add Alembic scaffolding + baseline migration

Introduce Alembic at v1. Migrations live inside the package
(decnet/web/db/migrations) so they ship with installs; alembic.ini at the
repo root drives the CLI. env.py is async and dual-backend, selecting the
engine from DECNET_DB_TYPE (mirroring db/factory.py) and reusing the app's
own connection when run programmatically.

The baseline captures all 39 tables. _BIG_TEXT round-trips as
Text().with_variant(MEDIUMTEXT, 'mysql'), so both backends get the right
column type from the migration. kd_digraph_simhash gains a sqlite BLOB
variant: BINARY(8) reflects as NUMERIC on SQLite and would otherwise trip
'alembic check' forever.

decnet/web/db/migrations/script.py.mako

@@ -0,0 +1,29 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+import sqlmodel  # SQLModel column types (AutoString, …) referenced by autogenerate
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision: str = ${repr(up_revision)}
+down_revision: Union[str, Sequence[str], None] = ${repr(down_revision)}
+branch_labels: Union[str, Sequence[str], None] = ${repr(branch_labels)}
+depends_on: Union[str, Sequence[str], None] = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    ${downgrades if downgrades else "pass"}