feat(documenso): version-aware field placement + void abstractions

Adds DOCUMENSO_API_VERSION env (default v1) plus per-port override.
Introduces placeFields, placeDefaultSignatureFields, and voidDocument
that hide v1 (per-field POST, pixel coords) vs v2 (bulk POST, percent +
fieldMeta) differences. cancelDocument now voids in Documenso first and
treats transient void failures as recoverable so the CRM stays the
system of record. 16 unit specs cover dispatch, layout math, idempotent
404, and v1 pixel conversion.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/env.ts

@@ -23,6 +23,7 @@ const envSchema = z.object({
   // Documenso
   DOCUMENSO_API_URL: z.string().url(),
   DOCUMENSO_API_KEY: z.string().min(1),
+  DOCUMENSO_API_VERSION: z.enum(['v1', 'v2']).default('v1'),
   DOCUMENSO_WEBHOOK_SECRET: z.string().min(16),
   DOCUMENSO_TEMPLATE_ID_EOI: z.coerce.number().int().positive().default(8),
   DOCUMENSO_CLIENT_RECIPIENT_ID: z.coerce.number().int().positive().default(192),