feat: add CSC AES/QES signing (v1 instance-wide config) (#2874)

Adds Cloud Signature Consortium (CSC) integration for AES/QES signing
against a configured TSP. v1 ships as instance-wide configuration via
environment variables, with per-envelope signature level selection,
license gating, and an OAuth-driven signing flow (capture + FIFO
signers, SAD session, blocking/in-progress recipient pages).

Includes signature level compatibility checks (role, signing order,
dictate next signer), envelope mutability assertions, Prisma migration
for signature level and CSC tables, and docs for the new signing
certificate options.

packages/lib/constants/app.ts

@@ -1,4 +1,6 @@
 import { env } from '@documenso/lib/utils/env';
+import { AppError, AppErrorCode } from '../errors/app-error';
+import { SignatureLevel, type TSignatureLevel } from '../types/signature-level';
 
 export const APP_DOCUMENT_UPLOAD_SIZE_LIMIT = Number(env('NEXT_PUBLIC_DOCUMENT_SIZE_UPLOAD_LIMIT')) || 50;
 
@@ -33,3 +35,54 @@ export const IS_AI_FEATURES_CONFIGURED = () => !!env('GOOGLE_VERTEX_PROJECT_ID')
 export const NEXT_PRIVATE_USE_PLAYWRIGHT_PDF = () => env('NEXT_PRIVATE_USE_PLAYWRIGHT_PDF') === 'true';
 
 export const NEXT_PRIVATE_SIGNING_TIMESTAMP_AUTHORITY = () => env('NEXT_PRIVATE_SIGNING_TIMESTAMP_AUTHORITY');
+
+/**
+ * Whether this Documenso instance is running in CSC (Cloud Signature Consortium) mode.
+ *
+ * CSC mode routes signing through a third-party Trust Service Provider for
+ * Advanced and Qualified Electronic Signatures (AES/QES). It is instance-wide
+ * and mutually exclusive with the other signing transports.
+ */
+export const IS_INSTANCE_CSC_MODE = (): boolean => {
+  if (typeof window === 'undefined') {
+    return env('NEXT_PRIVATE_SIGNING_TRANSPORT') === 'csc';
+  }
+
+  return env('NEXT_PUBLIC_SIGNING_TRANSPORT_IS_CSC') === 'true';
+};
+
+/**
+ * The default signature level applied to envelopes created on a CSC-mode
+ * instance when the caller doesn't specify one (or asks for `SES` and the
+ * resolver is in loose-coerce mode).
+ *
+ * Set via `NEXT_PRIVATE_SIGNING_CSC_SIGNATURE_LEVEL`; accepts `AES` or `QES`
+ * only; defaults to `AES` when unset. An explicit `AES`/`QES` request on
+ * envelope create still passes through unchanged — this constant only affects
+ * the coerced default.
+ *
+ * Throws on an invalid value rather than silently falling back. A typo here
+ * (e.g. `qes`) would otherwise silently downgrade qualified-tier instances
+ * to advanced-tier, which has legal consequences.
+ *
+ * Only consulted on CSC-mode instances. Non-CSC instances always default to
+ * `SES` regardless of this var.
+ */
+export const CSC_INSTANCE_SIGNATURE_LEVEL = (): TSignatureLevel => {
+  // Cast through `string | undefined` because shells can deliver
+  // `NEXT_PRIVATE_SIGNING_CSC_SIGNATURE_LEVEL=` as an empty string at runtime
+  // — the typed env signature narrows to `'AES' | 'QES' | undefined` only.
+  const value = env('NEXT_PRIVATE_SIGNING_CSC_SIGNATURE_LEVEL');
+
+  if (!value) {
+    return SignatureLevel.AES;
+  }
+
+  if (value !== SignatureLevel.AES && value !== SignatureLevel.QES) {
+    throw new AppError(AppErrorCode.NOT_SETUP, {
+      message: `NEXT_PRIVATE_SIGNING_CSC_SIGNATURE_LEVEL must be '${SignatureLevel.AES}' or '${SignatureLevel.QES}', got '${value}'.`,
+    });
+  }
+
+  return value;
+};