feat(email): add provider abstraction and HTML templates

Add pluggable email provider system supporting SMTP, Mailgun, and SendGrid
with factory pattern for runtime provider selection. Includes branded HTML
email templates for verification, password reset, MFA notifications, and
organization invites.

Also rebrands all email content from Gatehouse to Secuird, adds email
provider configuration options, and fixes duplicate log handlers in
development mode.

config/development.py

@@ -20,13 +20,13 @@ class DevelopmentConfig(BaseConfig):
     # Reduced bcrypt rounds for faster dev cycles
     BCRYPT_LOG_ROUNDS = 4
 
-    # Gatehouse React UI URL — OIDC authorize redirects here instead of showing raw HTML
+    # Secuird React UI URL — OIDC authorize redirects here instead of showing raw HTML
     OIDC_UI_URL = os.getenv("OIDC_UI_URL", "http://localhost:8080")
 
     # Add localhost:8080 (React UI) to CORS allowed origins for OIDC bridge endpoints
     CORS_ORIGINS = os.getenv(
         "CORS_ORIGINS",
-        "http://localhost:8080,http://localhost:3000,http://localhost:5173,https://ui.webauthn.local"
+        "http://192.168.50.124:8080,http://localhost:8080,http://localhost:3000,http://localhost:5173,https://ui.webauthn.local"
     ).split(",")
 
     # ── Email / SMTP ──────────────────────────────────────────────────────────
@@ -40,3 +40,11 @@ class DevelopmentConfig(BaseConfig):
     SMTP_USE_TLS = os.getenv("SMTP_USE_TLS", "").lower() == "true" if os.getenv("SMTP_USE_TLS") else int(os.getenv("SMTP_PORT", "1025")) not in (25, 1025)
     FROM_ADDRESS = os.getenv("FROM_ADDRESS", "noreply@gatehouse.local")
     EMAIL_FROM = FROM_ADDRESS  # alias
+
+    # Email Provider Configuration
+    EMAIL_PROVIDER = os.getenv("EMAIL_PROVIDER", "smtp").lower()
+    MAILGUN_API_KEY = os.getenv("MAILGUN_API_KEY") or None
+    MAILGUN_DOMAIN = os.getenv("MAILGUN_DOMAIN") or None
+    MAILGUN_API_URL = os.getenv("MAILGUN_API_URL", "https://api.mailgun.net/v3")
+    SENDGRID_API_KEY = os.getenv("SENDGRID_API_KEY") or None
+    SENDGRID_FROM_EMAIL = os.getenv("SENDGRID_FROM_EMAIL") or None