Feat: Rabattsystem in Rechnungserstellung integriert (Gründer/Referral)

- _get_discount_info() Hilfsfunktion in admin.py (Gründer 100%, Referral-Stufen 20/30/50%, von Gründer eingeladen 50%)
- list_upgrade_requests liefert discount_pct + discount_reason pro User
- GET /admin/users/{user_id}/discount Endpoint
- _handle_upgrade_invoices nutzt Rabatt für amount_net/discount_pct/after_disc + passende Notiz
- scheduler.py _create_renewal_invoice_draft: inline Rabattberechnung + korrekte Beträge
- admin.js: Discount-Badge in Upgrade-Card, data-Attribute am Invoice-Button, _discountNote(), discount_pct + notes im Modal vorbelegt

backend/routes/admin.py

@@ -1137,13 +1137,28 @@ async def list_upgrade_requests(user=Depends(require_admin)):
     with db() as conn:
         rows = conn.execute("""
             SELECT r.id, r.user_id, r.tier, r.message, r.created_at, r.fulfilled_at,
-                   u.name, u.email, u.billing_address
+                   u.name, u.email, u.billing_address,
+                   u.is_founder, u.is_founder_pending, u.referred_by,
+                   COALESCE((SELECT COUNT(*) FROM users WHERE referred_by=u.id), 0) AS referral_count
             FROM upgrade_requests r
             JOIN users u ON u.id = r.user_id
             ORDER BY r.fulfilled_at IS NOT NULL, r.created_at DESC
             LIMIT 100
         """).fetchall()
-    return [dict(r) for r in rows]
+        result = []
+        for r in rows:
+            d = dict(r)
+            d_info = _get_discount_info(conn, r["user_id"])
+            d["discount_pct"]    = d_info["discount_pct"]
+            d["discount_reason"] = d_info["reason"]
+            result.append(d)
+    return result
+
+
+@router.get("/users/{user_id}/discount")
+def get_user_discount(user_id: int, admin=Depends(require_admin)):
+    with db() as conn:
+        return _get_discount_info(conn, user_id)
 
 
 @router.post("/upgrade-requests/{req_id}/fulfill")
@@ -1268,6 +1283,35 @@ async def fulfill_upgrade_request(req_id: int, user=Depends(require_admin)):
     return {"ok": True, "tier": req["tier"], "user": req["name"]}
 
 
+def _get_discount_info(conn, user_id: int) -> dict:
+    """Berechnet Rabatt für einen User basierend auf Gründer-Status und Referrals."""
+    row = conn.execute(
+        """SELECT u.is_founder, u.is_founder_pending, u.referred_by,
+                  COALESCE((SELECT COUNT(*) FROM users WHERE referred_by=u.id), 0) AS referral_count
+           FROM users u WHERE u.id=?""",
+        (user_id,)
+    ).fetchone()
+    if not row:
+        return {"discount_pct": 0, "reason": None, "referral_count": 0}
+
+    if row["is_founder"] or row["is_founder_pending"]:
+        return {"discount_pct": 100, "reason": "founder", "referral_count": row["referral_count"]}
+
+    referred_by = row["referred_by"] or 0
+    if referred_by > 0:
+        referrer = conn.execute(
+            "SELECT is_founder, is_founder_pending FROM users WHERE id=?", (referred_by,)
+        ).fetchone()
+        if referrer and (referrer["is_founder"] or referrer["is_founder_pending"]):
+            return {"discount_pct": 50, "reason": "referred_by_founder", "referral_count": row["referral_count"]}
+
+    count = row["referral_count"]
+    for threshold, pct in [(50, 50), (20, 30), (10, 20)]:
+        if count >= threshold:
+            return {"discount_pct": pct, "reason": "referral", "referral_count": count}
+    return {"discount_pct": 0, "reason": None, "referral_count": count}
+
+
 async def _handle_upgrade_invoices(req: dict, new_tier_label: str):
     """Storniert offene Rechnungen des alten Tiers und legt neuen Entwurf an."""
     from routes.invoices import _next_invoice_number
@@ -1302,17 +1346,31 @@ async def _handle_upgrade_invoices(req: dict, new_tier_label: str):
         ).fetchone()
         billing_address = billing["billing_address"] if billing else None
 
+        disc_info    = _get_discount_info(conn, req["user_id"])
+        discount_pct = disc_info["discount_pct"]
+        discount_amt = round(price * discount_pct / 100, 2)
+        after_disc   = round(price - discount_amt, 2)
+
+        _AGB = "Jahresbeitrag gem. AGB. Bei vorzeitiger Kündigung keine anteilige Rückerstattung; Zugang bleibt bis Laufzeitende bestehen."
+        if disc_info["reason"] == "founder":
+            note = f"Gründer-Sonderkonditionen: {new_tier_label} kostenfrei als Dankeschön für deine Unterstützung als Gründer! {_AGB}"
+        elif disc_info["reason"] == "referred_by_founder":
+            note = f"Willkommen in der Gründer-Community! Als persönlich von einem Gründer eingeladenes Mitglied erhältst du dauerhaft {discount_pct}% Rabatt. {_AGB}"
+        elif disc_info["reason"] == "referral":
+            note = f"Herzlichen Dank für deine Unterstützung! Für {disc_info['referral_count']} geworbene Freunde erhältst du {discount_pct}% Rabatt. {_AGB}"
+        else:
+            note = f"{_AGB} (Upgrade von {req.get('old_tier','Standard')} auf {new_tier_label})"
+
         inv_number = _next_invoice_number(conn)
         conn.execute("""
             INSERT INTO invoices
                 (invoice_number, user_id, recipient_name, recipient_email, recipient_address,
                  description, service_period, amount_net, discount_pct, discount_amount,
                  amount_after_discount, tax_rate, tax_amount, amount_gross, notes)
-            VALUES (?,?,?,?,?,?,?,?,0,0,?,0,0,?,?)
+            VALUES (?,?,?,?,?,?,?,?,?,?,?,0,0,?,?)
         """, (
             inv_number, req["user_id"], req["name"], req["email"], billing_address,
-            description, period, price, price, price,
-            f"Jahresbeitrag gem. AGB. Bei vorzeitiger Kündigung keine anteilige Rückerstattung; Zugang bleibt bis Laufzeitende bestehen. (Upgrade von {req.get('old_tier','Standard')} auf {new_tier_label})",
+            description, period, price, discount_pct, discount_amt, after_disc, after_disc, note,
         ))
         invoice_id = conn.execute("SELECT last_insert_rowid()").fetchone()[0]
         conn.execute(