feat: add Demo Requests module

- New demo_requests table (SQLAlchemy model + Alembic-ready)
- Public POST /api/demo-requests endpoint for the TapTrack website form
- Super-admin GET/PUT/DELETE endpoints to manage leads
- DemoRequestsPage.vue with stats row, filterable table, status updates, notes modal
- Sidebar nav item and route registered

backend/app/models/demo_request.py

@@ -0,0 +1,25 @@
+import uuid
+from datetime import datetime, timezone
+from sqlalchemy import String, DateTime, Text
+from sqlalchemy.orm import Mapped, mapped_column
+from app.database import Base
+
+
+class DemoRequest(Base):
+    __tablename__ = "demo_requests"
+
+    id: Mapped[str] = mapped_column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
+    school_name: Mapped[str] = mapped_column(String(255), nullable=False)
+    contact_person: Mapped[str] = mapped_column(String(255), nullable=False)
+    phone_or_email: Mapped[str] = mapped_column(String(255), nullable=False)
+    notes: Mapped[str | None] = mapped_column(Text, nullable=True)
+    status: Mapped[str] = mapped_column(String(50), default="new", nullable=False)
+    # "new" | "contacted" | "converted" | "dismissed"
+    submitted_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        onupdate=lambda: datetime.now(timezone.utc),
+    )