Whiteboard Plugin v0.1.2

v0.1.2 — alembic compat shim for older Zou

Partial revert of v0.1.1. Newer Zou (current `main`) ships a shared Alembic template under `zou/app/utils/plugin_alembic_template/`, so plugins don't need their own `env.py`/`alembic.ini`/`script.py.mako`. But older Zou versions still invoke the plugin's `env.py` directly during `zou install-plugin`, so dropping these files broke install on those servers with:

```
ImportError: Can't find Python file .../plugins/whiteboard/migrations/env.py
```

This release restores those files so the plugin installs cleanly on both old and new Zou. They're harmless on current `main` — the shared template is used in preference.

Upgrade

```bash
zou install-plugin --path ~/kitsu-whiteboard-plugin.zip
```

Includes everything from v0.1.1:

• `table_args = {"extend_existing": True}` on `Board`
• `post_install` hook runs `npm install && npm run build` when `frontend/dist/` is missing
• Cross-platform Linux-compatible release ZIP
• README troubleshooting for external Postgres and the install-from-inside-dir recursion bug