Añade env de ingesta, SMTP y webhooks + dependencia nodemailer

Variables nuevas (todas opcionales vía zod, sin romper build/demo):
- FUNNEL_API_KEY: clave Bearer del EP de ingesta.
- SMTP_* + EMAIL_FROM: envío de email del presupuesto/enlace.
- PERFIL/WHATSAPP/WHATSAPP_START webhook URLs: señales al flujo externo.
- APP_URL: base para enlaces absolutos.
Helpers emailConfigurado()/perfilWebhookConfigurado()/whatsappWebhookConfigurado()/
whatsappStartConfigurado(). nodemailer como dep directa (stack: Email SMTP).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

mvp/b2c/package-lock.json

@@ -13,6 +13,7 @@
         "bcryptjs": "^3.0.3",
         "drizzle-orm": "^0.45.2",
         "next": "16.2.6",
+        "nodemailer": "^8.0.10",
         "postcss": "^8.5.15",
         "postgres": "^3.4.9",
         "react": "19.2.4",
@@ -24,6 +25,7 @@
       },
       "devDependencies": {
         "@types/node": "^20",
+        "@types/nodemailer": "^8.0.0",
         "@types/react": "^19",
         "@types/react-dom": "^19",
         "@vitest/coverage-v8": "^4.1.7",
@@ -2966,6 +2968,16 @@
         "undici-types": "~6.21.0"
       }
     },
+    "node_modules/@types/nodemailer": {
+      "version": "8.0.0",
+      "resolved": "https://registry.npmjs.org/@types/nodemailer/-/nodemailer-8.0.0.tgz",
+      "integrity": "sha512-fyf8jWULsCo0d0BuoQ75i6IeoHs47qcqxWc7yUdUcV0pOZGjUTTOvwdG1PRXUDqN/8A64yQdQdnA2pZgcdi+cA==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@types/node": "*"
+      }
+    },
     "node_modules/@types/react": {
       "version": "19.2.15",
       "resolved": "https://registry.npmjs.org/@types/react/-/react-19.2.15.tgz",
@@ -7201,6 +7213,15 @@
         "node": ">=18"
       }
     },
+    "node_modules/nodemailer": {
+      "version": "8.0.10",
+      "resolved": "https://registry.npmjs.org/nodemailer/-/nodemailer-8.0.10.tgz",
+      "integrity": "sha512-BLFuSth7QtHOkBzyqTehWWyub0NTRDuK2Q2SQfnGLsrJnzyU+Yeh4WpV1eZGuARFj1xQJHIdnTuJZLP+b9R1GQ==",
+      "license": "MIT-0",
+      "engines": {
+        "node": ">=6.0.0"
+      }
+    },
     "node_modules/normalize-svg-path": {
       "version": "1.1.0",
       "resolved": "https://registry.npmjs.org/normalize-svg-path/-/normalize-svg-path-1.1.0.tgz",