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/db/__init__.py

@@ -45,8 +45,7 @@ def get_connection(db_path: str | None = None) -> sqlite3.Connection:
 
 def _has_schema_version(conn: sqlite3.Connection) -> bool:
     try:
-        conn.execute("SELECT 1 FROM schema_version").fetchone()
-        return True
+        return conn.execute("SELECT 1 FROM schema_version LIMIT 1").fetchone() is not None
     except sqlite3.OperationalError:
         return False
 
@@ -75,8 +74,11 @@ def init_db(db_path: str | None = None) -> sqlite3.Connection:
             logger.info("Detected pre-M3 DB, rebuilding schema")
             conn.executescript("DROP TABLE IF EXISTS settings")
             conn.executescript(schema)
-conn.commit()
             conn.execute("INSERT OR REPLACE INTO schema_version (version) VALUES (1)")
+        else:
+            # M3-era tables present but no version stamped; stamp it.
+            conn.execute("INSERT OR REPLACE INTO schema_version (version) VALUES (1)")
+        conn.commit()
 
     return conn
 
@@ -115,6 +117,10 @@ def _reset_for_tests() -> None:
     global _conn, _current_path
     with _lock:
         if _conn is not None:
+            try:
+                _conn.execute("PRAGMA wal_checkpoint(TRUNCATE)")
+            except Exception:
+                pass
             try:
                 _conn.close()
             except Exception: