fix(db): run migrations on Chinese, Japanese and Korean Windows

alembic reads alembic.ini with encoding="locale". On Python 3.11 that is
the Windows ANSI code page even under PYTHONUTF8=1, which the desktop
shell sets, so the em dashes in the ini's comments raised
UnicodeDecodeError inside Config() on cp932/cp936/cp949/cp950 systems.
_run_alembic_upgrade treats that as "alembic unavailable": every startup
logged "alembic upgrade head skipped: 'cp949' codec can't decode byte
0xe2", never took the pre-migration backup, and never ran a migration --
including the data-healing ones (0006/0007) that the additive column
reconcile cannot replace.

The ini is now ASCII, with a note saying why, and a test parses it in
each of those code pages with the parser alembic builds and pins it
ASCII. Same Python 3.11 locale-decoding class as the .pth fix in #1795.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

alembic.ini

@@ -1,25 +1,29 @@
 # Alembic configuration for VoiceStudio.
 # Run from anywhere:  alembic -c <repo>/alembic.ini <command>
 # Default commands:
-#   alembic upgrade head         — apply all pending migrations
+#   alembic upgrade head         - apply all pending migrations
-#   alembic revision -m "…"      — create a new migration
+#   alembic revision -m "..."    - create a new migration
-#   alembic current              — show current schema version
+#   alembic current              - show current schema version
+#
+# Keep this file ASCII: alembic reads it in the locale code page, which on a
+# Chinese, Japanese or Korean Windows cannot decode UTF-8 punctuation
+# (tests/test_alembic_ini_locale.py).
 #
 # DB URL is resolved dynamically from core.config (env aware).
 # See backend/migrations/env.py.
 
 [alembic]
 # %(here)s = this file's directory. Alembic resolves bare relative paths
-# against the process CWD, not the ini — and the app doesn't always start
+# against the process CWD, not the ini - and the app doesn't always start
 # from the repo root (`tauri dev` runs the backend with
 # cwd=frontend/src-tauri), which made startup migrations die with
 # "Path doesn't exist: backend/migrations" the first time one was pending.
 script_location = %(here)s/backend/migrations
 prepend_sys_path = %(here)s/backend
 # Split multi-path options on os.pathsep, not the legacy space/comma/colon
-# set — a colon-split would shred "C:\..." absolute paths on Windows.
+# set - a colon-split would shred "C:\..." absolute paths on Windows.
 path_separator = os
-# sqlalchemy.url is set programmatically in env.py — do NOT set it here.
+# sqlalchemy.url is set programmatically in env.py - do NOT set it here.
 sqlalchemy.url =
 
 [loggers]

tests/test_alembic_ini_locale.py

@@ -0,0 +1,39 @@
+"""alembic.ini must parse in every Windows ANSI code page.
+
+alembic reads its ini with ``encoding="locale"`` (``alembic.util.compat.
+read_config_parser``). On Python 3.11 that is the Windows ANSI code page even
+with ``PYTHONUTF8=1``, which the desktop shell sets — so on a Chinese, Japanese
+or Korean Windows the em dashes in the ini's comments raised
+``UnicodeDecodeError`` inside ``Config()``, and every startup logged
+"alembic upgrade head skipped" and never ran a migration or took the
+pre-migration backup.
+"""
+from __future__ import annotations
+
+import configparser
+from pathlib import Path
+
+import pytest
+
+_INI = Path(__file__).resolve().parents[1] / "alembic.ini"
+
+
+@pytest.mark.parametrize("code_page", ["cp932", "cp936", "cp949", "cp950", "cp1252"])
+def test_alembic_ini_parses_in_the_windows_ansi_code_page(code_page):
+    # The same parser alembic.config.Config builds: `here` is its default.
+    parser = configparser.ConfigParser({"here": str(_INI.parent)})
+    assert parser.read(_INI, encoding=code_page) == [str(_INI)]
+    assert parser.get("alembic", "script_location").endswith("/backend/migrations")
+
+
+def test_alembic_ini_is_ascii_so_no_locale_can_break_it():
+    data = _INI.read_bytes()
+    offenders = [
+        f"line {n}: {line.decode('utf-8', 'replace').strip()}"
+        for n, line in enumerate(data.splitlines(), start=1)
+        if any(b > 0x7F for b in line)
+    ]
+    assert not offenders, (
+        "alembic.ini is read in the locale code page, so it must stay ASCII:\n  "
+        + "\n  ".join(offenders)
+    )