refactor(db_migrate): extract _apply_migrations_until helper; add edge-case tests

Issue 1 (refactor): run() and migrate_to_version() duplicated the inner
loop body — every change to the migration loop had to be applied in both
places. Pull the loop into _apply_migrations_until(engine, target_version):
when target_version is None, apply all pending; otherwise stop when the
file's version exceeds target. run() and migrate_to_version() are now
thin wrappers around the helper.

Issue 2 (tests): migrate_to_version() had no direct tests for its
edge-case behavior. Add 4 tests:
- idempotent: calling migrate_to_version(engine, N) twice == once
- lower-than-current: migrate_to_version(engine, 3) after N=13 is a no-op
- zero on fresh DB: migrate_to_version(engine, 0) is a no-op
- beyond-max: migrate_to_version(engine, 999) applies every migration

Tests use a fresh sa.Engine (not db.engine()) so they exercise the
target-version behavior from a clean user_version=0 starting point —
the conftest autouse fixture has already brought the module engine to
the latest version, which would mask the no-op branches.

backend/tests/test_db_migrate.py

@@ -587,4 +587,70 @@ def test_migration_0014_preserves_joint_fk_rows(tmp_path, monkeypatch):
         assert rows == [("CLM-J", "B-JOIN", "CLP-J", "B-JOIN-R")], (
             "matches batch_id + remittance_batch_id must be populated by 0014; "
             "got " + repr(rows)
-        )
+        )
+
+
+# =============================================================================
+# migrate_to_version: edge-case behavior
+# =============================================================================
+# These tests use a fresh ``sa.Engine`` (not ``db.engine()``) because the
+# autouse conftest fixture has already initialized the module engine at the
+# latest version. We need a clean engine that starts at ``user_version=0``
+# so that ``migrate_to_version(N)`` actually exercises the apply / no-op /
+# beyond-max branches.
+
+
+def test_migrate_to_version_idempotent(tmp_path, monkeypatch):
+    """Calling migrate_to_version(engine, N) twice is equivalent to calling once."""
+    from cyclone.db_migrate import migrate_to_version
+
+    engine = _fresh_engine(tmp_path)
+    migrate_to_version(engine, 5)
+    with engine.begin() as conn:
+        v1 = conn.exec_driver_sql("PRAGMA user_version").scalar()
+    migrate_to_version(engine, 5)
+    with engine.begin() as conn:
+        v2 = conn.exec_driver_sql("PRAGMA user_version").scalar()
+    assert v1 == v2 == 5
+
+
+def test_migrate_to_version_lower_than_current_is_noop(tmp_path, monkeypatch):
+    """migrate_to_version(engine, 3) after migrate_to_version(engine, 13) is a no-op."""
+    from cyclone.db_migrate import migrate_to_version
+
+    engine = _fresh_engine(tmp_path)
+    migrate_to_version(engine, 13)
+    with engine.begin() as conn:
+        v_after_13 = conn.exec_driver_sql("PRAGMA user_version").scalar()
+    migrate_to_version(engine, 3)  # lower, no-op
+    with engine.begin() as conn:
+        v_after_3 = conn.exec_driver_sql("PRAGMA user_version").scalar()
+    assert v_after_13 == v_after_3 == 13
+
+
+def test_migrate_to_version_zero_on_fresh_db(tmp_path, monkeypatch):
+    """migrate_to_version(engine, 0) on a fresh DB is a no-op."""
+    from cyclone.db_migrate import migrate_to_version
+
+    engine = _fresh_engine(tmp_path)
+    migrate_to_version(engine, 0)
+    with engine.begin() as conn:
+        v = conn.exec_driver_sql("PRAGMA user_version").scalar()
+    assert v == 0
+
+
+def test_migrate_to_version_beyond_max_applies_all(tmp_path, monkeypatch):
+    """migrate_to_version(engine, 999) applies every migration (equivalent to run)."""
+    import re
+    from cyclone.db_migrate import MIGRATIONS_DIR, migrate_to_version
+
+    engine = _fresh_engine(tmp_path)
+    migrate_to_version(engine, 999)
+    with engine.begin() as conn:
+        v = conn.exec_driver_sql("PRAGMA user_version").scalar()
+    # Should equal the highest version present in the migrations directory.
+    max_version = max(
+        int(re.match(r"^(\d+)", p.name).group(1))
+        for p in MIGRATIONS_DIR.glob("*.sql")
+    )
+    assert v == max_version