Implement Wix token persistence, refresh flow, and live API routes

[AJaySi]

Motivation

• Replace placeholder frontend-only Wix logic with server-backed token handling so routes can operate using persisted OAuth tokens and refresh flows.
• Ensure publish, categories, tags, status and disconnect endpoints work with real Wix API calls and surface proper connection/site metadata.
• Provide consistent HTTP error mapping for auth/permission/upstream failures.

Description

• Added helpers _get_current_user_id, _resolve_valid_wix_token, and _map_wix_error to resolve current user, load/refresh persisted tokens via WixOAuthService and WixService.refresh_access_token, and map errors to 401/403/5xx HTTP responses.
• Updated /connection/status and /status to return connected: true and live site_info/permission metadata using the resolved access token and wix_service.check_blog_permissions.
• Implemented /publish to resolve member_id, call WixService.create_blog_post(...), persist refreshed tokens when applicable, and return canonical fields post_id, url, and publish_state.
• Implemented /categories and /tags to proxy real responses from WixService.get_blog_categories and WixService.get_blog_tags using the resolved token.
• Implemented /disconnect to revoke stored token records for the current user via WixOAuthService.revoke_token and return a disconnected status.

Testing

• Ran python -m py_compile backend/api/wix_routes.py and it succeeded.
• Code was committed (f2f8d98) after local validation; no additional automated integration/unit tests were executed in this rollout.

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