fix(db): Migrationen ohne Fremdschlüsselprüfung und in einer Transaktion

Migration 0002 scheiterte auf einer Datenbank mit verknüpften Daten
(FOREIGN KEY constraint failed beim Neuaufbau von credentials) und
hinterließ einen halben Zustand, weil SQLite-DDL nicht transaktional lief.
Eigene Migrationsverbindung: foreign_keys=OFF, BEGIN IMMEDIATE,
transactional_ddl, foreign_key_check vor dem Commit. Tests mit echten,
verknüpften Daten und mit Abbruch mitten in der Migration.

update.sh: beim Zurückstufen keinen Changelog-Auszug.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_migrations.py

@@ -0,0 +1,136 @@
+"""Migrationen auf Datenbanken MIT Daten.
+
+Praxisbefund beim Update 0.1.1 → 0.2.0: Migration 0002 baut auf SQLite Tabellen neu
+auf (batch mode) und löscht dafür die alte Tabelle. Mit eingeschalteter
+Fremdschlüsselprüfung scheiterte das, sobald Anbindungen auf Zugangsdaten
+verwiesen – die Tests liefen bis dahin nur auf leeren Datenbanken. Und weil die
+Migration nicht in einer Transaktion lief, blieb ein halber Zustand zurück.
+"""
+
+from __future__ import annotations
+
+import sqlite3
+from pathlib import Path
+
+import pytest
+from alembic import command
+
+from netmover.db import alembic_config, upgrade
+
+
+def _seed(db: Path) -> None:
+    """Datenbank auf Stand 0001 mit verknüpften Daten, wie auf einem echten System."""
+    c = sqlite3.connect(db)
+    c.execute("PRAGMA foreign_keys=ON")
+    c.executescript(
+        """
+        INSERT INTO credentials (id, name, kind, username, secret_enc, created_at)
+            VALUES (1, 'mount:nas', 'password', 'scanner', 'gAAAA-x', '2026-10-10 20:00:00'),
+                   (2, 'mount:paperless', 'ssh_key', 'root', 'gAAAA-y', '2026-10-10 20:00:00');
+        INSERT INTO mounts (id, name, slug, type, server, share, mountpoint, cred_id,
+                            extra_options, auto_mount, enabled, state)
+            VALUES (1, 'NAS', 'nas', 'cifs', '10.0.10.6', 'Scans', '/mnt/netmover/nas', 1,
+                    '', 1, 1, 'mounted'),
+                   (2, 'paperless', 'paperless', 'sftp', '10.0.10.17', '/consume', NULL, 2,
+                    '', 0, 1, 'mounted');
+        INSERT INTO rules (id, name, enabled, src_mount_id, src_subpath, dst_mount_id, dst_path,
+                           pattern, pattern_type, recursive, interval_seconds, stability_seconds,
+                           min_age_seconds, op_timeout_seconds, action, on_collision, dry_run,
+                           quarantine_after, consecutive_failures, created_at)
+            VALUES (1, 'Scans → Paperless', 0, 1, '', 2, '', '*.pdf', 'glob', 0, 30, 5, 0, 120,
+                    'move', 'rename', 0, 3, 0, '2026-10-10 20:00:00');
+        INSERT INTO runs (id, rule_id, started_at, status, "trigger", dry_run, seen, moved,
+                          skipped, failed)
+            VALUES (1, 1, '2026-10-10 20:00:00', 'ok', 'manual', 1, 3, 3, 0, 0);
+        INSERT INTO run_items (run_id, src, status) VALUES (1, 'a.pdf', 'planned');
+        """
+    )
+    c.commit()
+    c.close()
+
+
+@pytest.fixture
+def db_0001(tmp_path: Path) -> tuple[str, Path]:
+    db = tmp_path / "alt.db"
+    url = f"sqlite:///{db}"
+    upgrade(url, "0001")
+    _seed(db)
+    return url, db
+
+
+def _state(db: Path) -> dict[str, object]:
+    c = sqlite3.connect(db)
+    try:
+        return {
+            "version": c.execute("SELECT version_num FROM alembic_version").fetchone()[0],
+            "tmp_tables": c.execute(
+                "SELECT name FROM sqlite_master WHERE name LIKE '_alembic_tmp%'"
+            ).fetchall(),
+            "cred_columns": [r[1] for r in c.execute("PRAGMA table_info(credentials)")],
+            "mounts": c.execute("SELECT name, cred_id FROM mounts ORDER BY id").fetchall(),
+            "rules": c.execute("SELECT name, src_mount_id, dst_mount_id FROM rules").fetchall(),
+            "run_items": c.execute("SELECT COUNT(*) FROM run_items").fetchone()[0],
+            "fk_violations": c.execute("PRAGMA foreign_key_check").fetchall(),
+        }
+    finally:
+        c.close()
+
+
+def test_upgrade_with_linked_data(db_0001: tuple[str, Path]) -> None:
+    url, db = db_0001
+    upgrade(url)
+    st = _state(db)
+    assert st["version"] == "0002"
+    assert st["tmp_tables"] == [] and st["fk_violations"] == []
+    assert "managed" in st["cred_columns"]  # type: ignore[operator]
+    assert st["mounts"] == [("NAS", 1), ("paperless", 2)]
+    assert st["rules"] == [("Scans → Paperless", 1, 2)]
+    assert st["run_items"] == 1
+    c = sqlite3.connect(db)
+    assert c.execute("SELECT managed FROM credentials ORDER BY id").fetchall() == [(0,), (0,)]
+    assert c.execute("SELECT username FROM mounts ORDER BY id").fetchall() == [(None,), (None,)]
+    c.close()
+
+
+def test_downgrade_with_linked_data(db_0001: tuple[str, Path]) -> None:
+    url, db = db_0001
+    upgrade(url)
+    command.downgrade(alembic_config(url), "0001")
+    st = _state(db)
+    assert st["version"] == "0001" and st["fk_violations"] == []
+    assert st["mounts"] == [("NAS", 1), ("paperless", 2)]
+
+
+def test_failed_migration_leaves_database_untouched(
+    db_0001: tuple[str, Path], monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """Bricht eine Migration mittendrin ab, muss alles beim Alten bleiben – keine
+    halb umgebauten Tabellen, kein _alembic_tmp-Rest, Version unverändert."""
+    from alembic.ddl.impl import DefaultImpl
+
+    url, db = db_0001
+    before = _state(db)
+    real_drop = DefaultImpl.drop_table
+
+    def boom(self: DefaultImpl, *a: object, **kw: object) -> None:
+        raise RuntimeError("Absturz mitten in der Migration")
+
+    monkeypatch.setattr(DefaultImpl, "drop_table", boom)
+    with pytest.raises(RuntimeError, match="Absturz"):
+        upgrade(url)
+    assert _state(db) == before
+
+    monkeypatch.setattr(DefaultImpl, "drop_table", real_drop)
+    upgrade(url)  # danach klappt es ohne Handarbeit
+    assert _state(db)["version"] == "0002"
+
+
+def test_app_engine_still_enforces_foreign_keys(db_url: str) -> None:
+    from sqlalchemy import text
+
+    from netmover.db import make_engine
+
+    engine = make_engine(db_url)
+    with engine.connect() as conn:
+        assert conn.execute(text("PRAGMA foreign_keys")).scalar() == 1
+    engine.dispose()