fix(db): schema_version drift detection + rebuild for pre-M3 volumes

- Add schema_version table; init_db detects a pre-M3/unknown DB (no version
  row, or settings table missing the value column) and rebuilds to the clean
  M3 schema, then stamps version 1
- _has_schema_version checks for a real row (empty table != versioned)
- _reset_for_tests checkpoints WAL before close (Windows handle release)
- TDD: test_schema_drift seeds the M2 settings shape, asserts rebuild + version

Fixes live 500 'no such column: value' on the UAT host (stale M2 slab_db volume).
Gates: check_requirements + check_coverage exit 0; pytest 14 passed.

backend/tests/test_schema_drift.py

@@ -1,37 +1,55 @@
 import os
-import tempfile
+import shutil
 import sqlite3
+import tempfile
+
+import pytest
 
 import backend.db as db_mod
 
 
+@pytest.fixture
+def drift_db_dir():
+    # Persistent temp dir; on Windows the global WAL connection can outlive the
+    # test frame, so we avoid TemporaryDirectory's immediate rmtree race and clean
+    # up best-effort after the handle is released.
+    tmp = tempfile.mkdtemp(prefix="slab_drift_")
+    yield tmp
+    db_mod._reset_for_tests()
+    shutil.rmtree(tmp, ignore_errors=True)
+
+
 
 def _setup_temp_db_with_m2_schema(tmp_dir: str) -> str:
     db_path = os.path.join(tmp_dir, "test.db")
+    # M2-era settings shape: NO `value` column (this is the drift signature).
+    # Use a raw connection (DELETE journal) exactly as an old M2 db would be on disk.
     conn = sqlite3.connect(db_path)
-    # create old settings table
-    conn.execute("CREATE TABLE settings (key TEXT PRIMARY KEY, value TEXT, value_enc TEXT)")
+    conn.execute("CREATE TABLE settings (key TEXT PRIMARY KEY, value_enc BLOB, updated_at TEXT)")
+    conn.execute("INSERT INTO settings (key, value_enc, updated_at) VALUES ('woo_wp_username', X'00', '2026-01-01')")
     conn.commit()
     conn.close()
+    # Force any journal sidecars to merge before the WAL-mode factory opens the file.
+    for suffix in ("-journal", "-wal", "-shm"):
+        try:
+            os.remove(db_path + suffix)
+        except OSError:
+            pass
     return db_path
 
 
-def test_drift_detection_and_rebuild(monkeypatch):
-    with tempfile.TemporaryDirectory() as tmp:
-        db_path = _setup_temp_db_with_m2_schema(tmp)
-        monkeypatch.setenv("SLAB_DB_PATH", db_path)
-        # init_db should rebuild
-        conn = db_mod.init_db()
-        # check schema_version exists
-        cur = conn.execute("SELECT version FROM schema_version").fetchone()
-        assert cur is not None
-        assert cur[0] == 1
-        # check settings has columns value and value_enc
-        info = conn.execute("PRAGMA table_info(settings)").fetchall()
-        cols = {row[1] for row in info}
-        assert "value" in cols
-        assert "value_enc" in cols
-        # also ensure old settings table dropped and recreated
-        # no other data
-        rows = conn.execute("SELECT key FROM settings").fetchall()
-        assert rows == []
+def test_drift_detection_and_rebuild(monkeypatch, drift_db_dir):
+    db_path = _setup_temp_db_with_m2_schema(drift_db_dir)
+    monkeypatch.setenv("SLAB_DB_PATH", db_path)
+    # init_db must detect pre-M3 drift and rebuild to the M3 settings shape
+    conn = db_mod.init_db()
+    cur = conn.execute("SELECT version FROM schema_version").fetchone()
+    assert cur is not None
+    assert cur[0] == 1
+    info = conn.execute("PRAGMA table_info(settings)").fetchall()
+    cols = {row[1] for row in info}
+    assert "value" in cols
+    assert "value_enc" in cols
+    # M2 rows are disposable (M3 supersedes M2 on this dev volume)
+    rows = conn.execute("SELECT key FROM settings").fetchall()
+    assert rows == []