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/README

@@ -0,0 +1 @@
+Generic single-database configuration.

decnet/web/db/migrations/env.py

@@ -0,0 +1,90 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+"""Alembic environment — async, dual-backend (sqlite | mysql).
+
+Two entry shapes:
+
+* **Programmatic** (app boot): :func:`decnet.web.db.migrate.run_migrations`
+  passes the app's own sync ``Connection`` via ``config.attributes`` so the
+  upgrade rides the existing engine — no second connection, no extra driver.
+* **Standalone** (``alembic`` CLI: autogenerate, upgrade, history): builds its
+  own async engine from ``DECNET_DB_TYPE``, mirroring ``db/factory.py``.
+"""
+from __future__ import annotations
+
+import asyncio
+import os
+from logging.config import fileConfig
+
+from alembic import context
+from sqlalchemy.engine import Connection
+from sqlmodel import SQLModel
+
+# Importing the models package registers every table on SQLModel.metadata,
+# which is what autogenerate diffs against.
+import decnet.web.db.models  # noqa: F401
+
+config = context.config
+
+# Standalone CLI runs configure logging from alembic.ini; the programmatic
+# path builds a Config with no file, so guard on it.
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+target_metadata = SQLModel.metadata
+
+
+def _build_async_engine():
+    """Standalone-only: pick an async engine the way db/factory.py does."""
+    db_type = os.environ.get("DECNET_DB_TYPE", "sqlite").lower()
+    if db_type == "sqlite":
+        from decnet.config import _ROOT
+        from decnet.web.db.sqlite.database import get_async_engine as sqlite_engine
+        db_path = os.environ.get("DECNET_DB_PATH", str(_ROOT / "decnet.db"))
+        return sqlite_engine(db_path)
+    if db_type == "mysql":
+        from decnet.web.db.mysql.database import get_async_engine as mysql_engine
+        return mysql_engine()
+    raise ValueError(f"Unsupported database type: {db_type}")
+
+
+def _configure_and_run(connection: Connection) -> None:
+    context.configure(
+        connection=connection,
+        target_metadata=target_metadata,
+        # SQLite can't ALTER in place; batch mode rewrites the table so future
+        # migrations (drop/alter column) work on both backends.
+        render_as_batch=connection.dialect.name == "sqlite",
+        compare_type=True,
+    )
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+async def _run_standalone() -> None:
+    engine = _build_async_engine()
+    async with engine.connect() as connection:
+        await connection.run_sync(_configure_and_run)
+    await engine.dispose()
+
+
+def run_migrations_online() -> None:
+    connection = config.attributes.get("connection", None)
+    if connection is not None:
+        # Programmatic: app handed us a live sync Connection (via run_sync).
+        _configure_and_run(connection)
+    else:
+        asyncio.run(_run_standalone())
+
+
+if context.is_offline_mode():
+    # Offline (--sql) mode: emit DDL without a DB. Cheap to support and keeps
+    # `alembic upgrade head --sql` working for operators who want to review SQL.
+    context.configure(
+        url=os.environ.get("DECNET_DB_URL"),
+        target_metadata=target_metadata,
+        literal_binds=True,
+    )
+    with context.begin_transaction():
+        context.run_migrations()
+else:
+    run_migrations_online()

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"}

decnet/web/db/models/attackers.py

@@ -13,7 +13,7 @@ from datetime import datetime, timezone
 from typing import Any, List, Optional
 
 from pydantic import BaseModel
-from sqlalchemy import BINARY, Column, Text, UniqueConstraint
+from sqlalchemy import BINARY, Column, LargeBinary, Text, UniqueConstraint
 from sqlmodel import Field, SQLModel
 
 from ._base import _BIG_TEXT
@@ -238,10 +238,18 @@ class AttackerIdentity(SQLModel, table=True):
     # registry); this column is the rollup the (future) attribution
     # engine will write into so the federation gossip layer
     # has one identity-level fingerprint to compare across operators.
-    # BINARY(8) so MySQL can index without a prefix length.
+    # BINARY(8) so MySQL can index without a prefix length. SQLite has no
+    # fixed-width binary type (BINARY → NUMERIC affinity, which reflects back
+    # as NUMERIC and trips `alembic check`), so use a BLOB variant there —
+    # bytes round-trip identically and the type matches what SQLite reports.
     kd_digraph_simhash: Optional[bytes] = Field(
         default=None,
-        sa_column=Column("kd_digraph_simhash", BINARY(8), nullable=True, index=True),
+        sa_column=Column(
+            "kd_digraph_simhash",
+            BINARY(8).with_variant(LargeBinary(), "sqlite"),
+            nullable=True,
+            index=True,
+        ),
     )
     # Soft-merge audit trail. When the clusterer collapses two
     # identities, the loser's row stays in place with this set to the