feat: wire MailService into notification dispatch pipeline (Stage 3)

Unifies the two previously independent email subsystems — MailService
(net/smtp transport) and NotificationService (HTTP-based providers) —
so email can participate in the notification dispatch pipeline.

Key changes:
- SendEmail signature updated to accept context.Context and []string
  recipients to enable timeout propagation and multi-recipient dispatch
- NotificationService.dispatchEmail() wires MailService as a first-class
  provider type with IsConfigured() guard and 30s context timeout
- 'email' added to isSupportedNotificationProviderType() and
  supportsJSONTemplates() returns false for email (plain/HTML only)
- settings_handler.go test-email endpoint updated to new SendEmail API
- Frontend: 'email' added to provider type union in notifications.ts,
  Notifications.tsx shows recipient field and hides URL/token fields for
  email providers
- All existing tests updated to match new SendEmail signature
- New tests added covering dispatchEmail paths, IsConfigured guards,
  recipient validation, and context timeout behaviour

Also fixes confirmed false-positive CodeQL go/email-injection alerts:
- smtp.SendMail, sendSSL w.Write, and sendSTARTTLS w.Write sites now
  carry inline codeql[go/email-injection] annotations as required by the
  CodeQL same-line suppression spec; preceding-line annotations silently
  no-op in current CodeQL versions
- auth_handler.go c.SetCookie annotated for intentional Secure=false on
  local non-HTTPS loopback (go/cookie-secure-not-set warning only)

Closes part of #800

frontend/src/api/notifications.ts

@@ -1,6 +1,6 @@
 import client from './client';
 
-export const SUPPORTED_NOTIFICATION_PROVIDER_TYPES = ['discord', 'gotify', 'webhook'] as const;
+export const SUPPORTED_NOTIFICATION_PROVIDER_TYPES = ['discord', 'gotify', 'webhook', 'email'] as const;
 export type SupportedNotificationProviderType = (typeof SUPPORTED_NOTIFICATION_PROVIDER_TYPES)[number];
 const DEFAULT_PROVIDER_TYPE: SupportedNotificationProviderType = 'discord';