fix: resolve open risks — invoice race condition, SMTP config, workflow seeds

- billing/service.py: pg_advisory_xact_lock on invoice_number_seq per year
  → prevents duplicate INV-YYYY-NNNN under concurrent requests
- admin.py: SMTP settings in system_settings (smtp_host/port/user/password/
  from_address/enabled) with GET+PUT support; seed-workflows endpoint creates
  4 standard workflow definitions (still-cycles, still-eevee, turntable,
  multi-angle) idempotently
- notifications/service.py: send_email_notification_stub now sends real
  SMTP email via smtplib when smtp_enabled=true in system_settings
- Admin.tsx: SMTP settings panel (host/port/user/password/from + enable
  toggle, save button); Seed Standard Workflows maintenance button
- Upload.tsx: fix TS error — title→aria-label on Lucide icons
- Admin.tsx Settings type: add render_backend/flamenco_* fields (TS fix)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/domains/notifications/service.py

@@ -112,11 +112,64 @@ def send_email_notification_stub(
     subject: str,
     body: str,
 ) -> None:
-    """Email notification stub — logs only, email sending not yet active."""
-    logger.info(
-        "[EMAIL STUB] Would send email to user=%s event=%s subject=%s",
-        to_user_id, event_type, subject
-    )
+    """Send email notification via SMTP if configured and enabled, else log only."""
+    engine = _get_engine()
+    try:
+        from sqlalchemy.orm import Session
+        from app.models.system_setting import SystemSetting
+        with Session(engine) as s:
+            rows = s.execute(
+                __import__("sqlalchemy").select(SystemSetting).where(
+                    SystemSetting.key.in_(["smtp_enabled", "smtp_host", "smtp_port", "smtp_user", "smtp_password", "smtp_from_address"])
+                )
+            ).scalars().all()
+            cfg = {r.key: r.value for r in rows}
+    except Exception:
+        cfg = {}
+
+    smtp_enabled = cfg.get("smtp_enabled", "false").lower() == "true"
+    smtp_host = cfg.get("smtp_host", "")
+
+    if not smtp_enabled or not smtp_host:
+        logger.info(
+            "[EMAIL STUB] Would send email to user=%s event=%s subject=%s (smtp disabled)",
+            to_user_id, event_type, subject
+        )
+        return
+
+    # Resolve to_address from user record
+    to_address: str | None = None
+    try:
+        from sqlalchemy.orm import Session
+        from app.models.user import User
+        with Session(engine) as s:
+            u = s.get(User, to_user_id) if to_user_id else None
+            if u:
+                to_address = u.email
+    except Exception:
+        pass
+
+    if not to_address:
+        logger.warning("[EMAIL] Could not resolve email for user=%s", to_user_id)
+        return
+
+    try:
+        import smtplib
+        from email.mime.text import MIMEText
+        msg = MIMEText(body, "plain", "utf-8")
+        msg["Subject"] = subject
+        msg["From"] = cfg.get("smtp_from_address") or cfg.get("smtp_user", "noreply@schaeffler.com")
+        msg["To"] = to_address
+        port = int(cfg.get("smtp_port", "587"))
+        with smtplib.SMTP(smtp_host, port) as smtp:
+            smtp.ehlo()
+            smtp.starttls()
+            if cfg.get("smtp_user") and cfg.get("smtp_password"):
+                smtp.login(cfg["smtp_user"], cfg["smtp_password"])
+            smtp.sendmail(msg["From"], [to_address], msg.as_string())
+        logger.info("[EMAIL] Sent to %s event=%s", to_address, event_type)
+    except Exception as exc:
+        logger.error("[EMAIL] Failed to send to %s: %s", to_address, exc)
 
 
 async def get_notification_configs(db: AsyncSession, user_id: uuid.UUID) -> list: