Provider-Agnostic Backend

## Problem

BluePLM is currently tightly coupled to Supabase services. Currently, there are ~557 direct Supabase client calls in the codebase. This vendor-locks the backend to use Supabase (which is not a big deal bc it is open-source).

## Goal

Enable BluePLM deployments with customer-owned infrastructure while preserving real-time collaboration features.

| Component | Supabase (default) | Alternative |
|-----------|-------------------|------------------------|
| **Auth** | Supabase Auth | OIDC (Keycloak, Auth0, Okta, Azure AD) |
| **Database** | Supabase Postgres | Standard PostgreSQL 14+ |
| **Storage** | Supabase Storage | S3-compatible (MinIO, AWS S3, GCS) |
| **Realtime** | Supabase Realtime | WebSocket server + pg_notify |