Add SMTP security mode selector with SSL/STARTTLS/none options and automatic port switching

Replaced boolean `smtp_use_tls` with explicit `smtp_security` enum field supporting "none", "starttls", and "ssl" modes. Added database migration to create `smtp_security` column with "starttls" default. Updated mailer to use SMTP_SSL client for direct SSL connections on port 465 and SMTP with STARTTLS for port 587. Modified admin settings UI to show dropdown selector with encryption options and auto-adjust

backend/app/main.py

@@ -88,6 +88,14 @@ Very small, pragmatic auto-migration (no alembic).
 - supports old schema (join_code/chip_code) and new schema (code/chip)
 """
 
+    # --- app settings: explicit SMTP security mode (none/starttls/ssl) ---
+    if not _has_column(db, "app_settings", "smtp_security"):
+        try:
+            db.execute(text("ALTER TABLE app_settings ADD COLUMN smtp_security VARCHAR DEFAULT 'starttls'"))
+            db.commit()
+        except Exception:
+            db.rollback()
+
     # --- users.display_name ---
     if not _has_column(db, "users", "display_name"):
         try: