feat(backend): add standalone data plane

Replace the Supabase application data plane with the Node/PostgreSQL API, migrate mobile, admin, web, and MCP callers, add controlled cutover tooling, and define hardened Dokploy infrastructure.

apps/mcp/README.md

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+# Sauci administrative MCP
+
+The MCP service exposes 46 internal tools through eight groups: content (12),
+users (7), moderation (5), redemption codes (5), administrators (5), analytics
+(4), configuration (4), and feedback (4).
+
+MCP does not connect to PostgreSQL, Supabase data APIs, or object storage. Tool
+requests are translated to the standalone API's `/v1/admin/*` contract. That
+API is responsible for resource allowlisting, permission checks, transactions,
+and mutation audit evidence. Relationship-shaped results required by existing
+tools are assembled from allowlisted API queries.
+
+## Environment
+
+- `SAUCI_MCP_API_KEY`: bearer credential accepted from MCP callers.
+- `SAUCI_ADMIN_API_URL`: standalone API base URL. Cleartext HTTP is rejected
+  except for loopback development.
+- `SAUCI_ADMIN_API_TOKEN`: opaque API service credential. The API maps it to
+  the active administrator configured by `ADMIN_API_SERVICE_USER_ID`.
+
+The two bearer credentials serve different trust boundaries and must not be
+reused. Missing admin API configuration fails when any data tool is invoked;
+missing MCP caller authentication fails closed at the HTTP boundary.