feat(email): SMTP mailer behind the EmailService seam (EMAIL_PROVIDER=smtp); Resend stays the hosted default (#1746)

SmtpEmailService (nodemailer 9.0.5, exact-pinned) behind the existing EmailService seam. Provider resolution: EMAIL_PROVIDER wins, else RESEND_API_KEY selects Resend (hosted byte-identical), else SMTP_HOST selects SMTP. From header is built exactly like the Resend service after #1956 (no 'via <app>', fromAddress honored, platform-sender retry). STARTTLS is required by default (requireTLS) with SMTP_REQUIRE_TLS=false as an explicit opt-out for a plaintext LAN relay. Docs, env examples and the generated extension registry updated.

lib/errors/structured-errors.ts

@@ -1070,10 +1070,10 @@ const INVOICE: Record<string, StructuredErrorEntry> = {
   INVOICE_SEND_EMAIL_NOT_CONFIGURED: {
     httpStatus: 503,
     message_sv:
-      'E-posttjänsten är inte konfigurerad. Kontrollera att RESEND_API_KEY och RESEND_FROM_EMAIL är satta.',
+      'E-posttjänsten är inte konfigurerad. Kontrollera att RESEND_API_KEY och RESEND_FROM_EMAIL är satta (eller SMTP_HOST och SMTP_FROM_EMAIL med EMAIL_PROVIDER=smtp).',
     message_en: 'Email service is not configured.',
     remediation: {
-      description: 'Set RESEND_API_KEY and RESEND_FROM_EMAIL in the deployment environment.',
+      description: 'Set RESEND_API_KEY and RESEND_FROM_EMAIL (or SMTP_HOST and SMTP_FROM_EMAIL with EMAIL_PROVIDER=smtp) in the deployment environment.',
     },
   },
   INVOICE_SEND_NO_CUSTOMER_EMAIL: {

lib/extensions/_generated/sector-definitions.ts

@@ -32,13 +32,13 @@ export const EXTENSION_DEFINITIONS: Record<string, ExtensionDefinition[]> = {
     },
     {
           "slug": "email",
-          "name": "E-post (Resend)",
+          "name": "E-post",
           "sector": "general",
           "category": "operations",
           "icon": "Mail",
           "dataPattern": "core",
           "description": "Skicka fakturor och påminnelser via e-post",
-          "longDescription": "Aktiverar e-postfunktioner: skicka fakturor till kunder, automatiska betalningspåminnelser enligt valt schema, och e-postmeddelanden. Kräver ett Resend-konto med verifierad domän.",
+          "longDescription": "Aktiverar e-postfunktioner: skicka fakturor till kunder, automatiska betalningspåminnelser enligt valt schema, och e-postmeddelanden. Kräver ett Resend-konto med verifierad domän eller en egen SMTP-server (EMAIL_PROVIDER=smtp).",
           "readsCoreTables": [
                 "invoices",
                 "customers",