Convert Facebook importers to BaseImporter classes (facebook_page, facebook_posts)

[aquarion]

Convert imports/facebook_page.py and imports/facebook_posts.py to class-based importers. These share the same Facebook OAuth flow and should probably share a FacebookBaseImporter or common auth helper.

Complexity

• Custom OAuth flow using CodeFetcher9000 or manual PIN entry
• Token stored as pickle in credentials dir
• Token expiry checking and user warnings
• facebook_posts.py has privacy filter logic (_load_filters, _post_is_visible)
• Uses graph_get() helper for API calls

Suggested approach

1. Create a shared _FacebookAuth mixin or FacebookBaseImporter handling token load/refresh
2. FacebookPageImporter and FacebookPostsImporter extend it
3. Move privacy filter logic into FacebookPostsImporter
4. Replace pickle token storage with read_token_file/write_token_file from oauth_utils (or keep pickle if the token format requires it)
5. Wire up pyproject.toml entry points
6. Delete old imports/facebook_*.py