feat: add per-user trial control in admin panel (backend + UI)

backend/main.py

@@ -1470,6 +1470,7 @@ class AdminUserPatch(BaseModel):
     school: Optional[str] = None
     city: Optional[str] = None
     state: Optional[str] = None
+    trial_days: Optional[int] = None  # number of trial days to grant from now (0 = remove trial)
 
 @app.get("/api/admin/students", tags=["Admin"])
 def list_all_students(session: Session = Depends(get_session), current_user: models.User = Depends(get_current_user)):
@@ -1608,7 +1609,7 @@ def get_student_detail(user_id: UUID, session: Session = Depends(get_session), c
 
 @app.patch("/api/admin/students/{user_id}", tags=["Admin"])
 def patch_student(user_id: UUID, payload: AdminUserPatch, session: Session = Depends(get_session), current_user: models.User = Depends(get_current_user)):
-    """Admin update: tier, name, streak, exam, location."""
+    """Admin update: tier, name, streak, exam, location, trial."""
     user = session.get(models.User, user_id)
     if not user:
         raise HTTPException(status_code=404, detail="Student not found")
@@ -1619,10 +1620,15 @@ def patch_student(user_id: UUID, payload: AdminUserPatch, session: Session = Dep
     if payload.school is not None: user.school = payload.school
     if payload.city is not None: user.city = payload.city
     if payload.state is not None: user.state = payload.state
+    if payload.trial_days is not None:
+        if payload.trial_days <= 0:
+            user.trial_end = None  # remove trial
+        else:
+            user.trial_end = datetime.utcnow() + timedelta(days=payload.trial_days)
     session.add(user)
     session.commit()
     session.refresh(user)
-    return {"status": "ok", "user_id": str(user.id), "tier": user.tier.value}
+    return {"status": "ok", "user_id": str(user.id), "tier": user.tier.value, "trial_end": user.trial_end.isoformat() if user.trial_end else None}
 
 # --- BILLING & PAYMENTS ---