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.

.gitignore

@@ -19,6 +19,8 @@ decnet-topology-*-compose.yml
 .docker/
 decnet-state.json
 *.ini
+# tracked: Alembic CLI config (migrations live in decnet/web/db/migrations)
+!alembic.ini
 decnet.log*
 *.loggy
 *.nmap