fix: SQLite migration 0003 FK drop fails on file-backed DBs (MB-2)

Add naming_convention to Base.metadata so Alembic batch-mode reflection
can match unnamed FK constraints created by migration 0002. Pass
naming_convention and render_as_batch=True through env.py online config.

Add migration roundtrip tests (upgrade/downgrade/cycle from baseline).

Amp-Thread-ID: https://ampcode.com/threads/T-019ce466-1683-7471-b6e1-bb078223cda0
Co-authored-by: Amp <amp@ampcode.com>

tests-unit/app_test/test_migrations.py

@@ -0,0 +1,57 @@
+"""Test that Alembic migrations run cleanly on a file-backed SQLite DB.
+
+This catches problems like unnamed FK constraints that prevent batch-mode
+drop_constraint from working on real SQLite files (see MB-2).
+
+Migrations 0001 and 0002 are already shipped, so we only exercise
+upgrade/downgrade for 0003+.
+"""
+
+import os
+
+import pytest
+from alembic import command
+from alembic.config import Config
+
+
+# Oldest shipped revision — we upgrade to here as a baseline and never
+# downgrade past it.
+_BASELINE = "0002_merge_to_asset_references"
+
+
+def _make_config(db_path: str) -> Config:
+    root = os.path.join(os.path.dirname(__file__), "../..")
+    config_path = os.path.abspath(os.path.join(root, "alembic.ini"))
+    scripts_path = os.path.abspath(os.path.join(root, "alembic_db"))
+
+    cfg = Config(config_path)
+    cfg.set_main_option("script_location", scripts_path)
+    cfg.set_main_option("sqlalchemy.url", f"sqlite:///{db_path}")
+    return cfg
+
+
+@pytest.fixture
+def migration_db(tmp_path):
+    """Yield an alembic Config pre-upgraded to the baseline revision."""
+    db_path = str(tmp_path / "test_migration.db")
+    cfg = _make_config(db_path)
+    command.upgrade(cfg, _BASELINE)
+    yield cfg
+
+
+def test_upgrade_to_head(migration_db):
+    """Upgrade from baseline to head must succeed on a file-backed DB."""
+    command.upgrade(migration_db, "head")
+
+
+def test_downgrade_to_baseline(migration_db):
+    """Upgrade to head then downgrade back to baseline."""
+    command.upgrade(migration_db, "head")
+    command.downgrade(migration_db, _BASELINE)
+
+
+def test_upgrade_downgrade_cycle(migration_db):
+    """Full cycle: upgrade → downgrade → upgrade again."""
+    command.upgrade(migration_db, "head")
+    command.downgrade(migration_db, _BASELINE)
+    command.upgrade(migration_db, "head")