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/api/drizzle/0001_couples.sql

@@ -0,0 +1,14 @@
+CREATE TABLE IF NOT EXISTS "couples" (
+	"id" uuid PRIMARY KEY NOT NULL,
+	"invite_code" text NOT NULL,
+	"created_at" timestamp with time zone DEFAULT now() NOT NULL,
+	CONSTRAINT "couples_invite_code_unique" UNIQUE("invite_code"),
+	CONSTRAINT "couples_invite_code_format" CHECK ("invite_code" ~ '^[A-Z0-9]{8}$')
+);
+--> statement-breakpoint
+DO $$ BEGIN
+ ALTER TABLE "profiles" ADD CONSTRAINT "profiles_couple_id_couples_id_fk"
+ FOREIGN KEY ("couple_id") REFERENCES "couples"("id") ON DELETE SET NULL;
+EXCEPTION
+ WHEN duplicate_object THEN null;
+END $$;