Feature: Upgrade-Anfragen-System — User-Flow + Admin-Panel (SW by-v920)

- DB: upgrade_requests-Tabelle (user_id, tier, message, fulfilled_at)
- POST /api/upgrade-request: Anfrage speichern + Admin-Benachrichtigungsmail
- GET/POST /api/admin/upgrade-requests[/{id}/fulfill]: Admin-Endpunkte
  — fulfill setzt subscription_tier + sendet Bestätigungsmail an User
- action-items: upgrades_pending zählt offene Anfragen → Badge im Admin
- Admin-Tab "Upgrades": Tabelle offener/erledigter Anfragen, Freischalten-Button
  mit Confirm-Modal, automatischer Tier-Setzung und Bestätigungsmail
- Settings: Upgrade-Modal sendet echte API-Anfrage statt nur mailto
  — doppelte Anfrage wird erkannt (already:true → Toast statt Fehler)
- api.js: API.auth.upgradeRequest(tier, message) hinzugefügt
- SW by-v920, APP_VER 920

backend/database.py

@@ -2341,6 +2341,24 @@ def _migrate(conn_factory):
         except Exception:
             pass
 
+        # upgrade_requests: Abo-Upgrade-Anfragen von Nutzern
+        try:
+            conn.execute("""
+                CREATE TABLE IF NOT EXISTS upgrade_requests (
+                    id           INTEGER PRIMARY KEY AUTOINCREMENT,
+                    user_id      INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+                    tier         TEXT    NOT NULL,
+                    message      TEXT,
+                    created_at   TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ','now')),
+                    fulfilled_at TEXT,
+                    fulfilled_by INTEGER REFERENCES users(id)
+                )
+            """)
+            conn.execute("CREATE INDEX IF NOT EXISTS idx_upgrade_req_pending ON upgrade_requests(fulfilled_at, created_at DESC)")
+            logger.info("Migration: upgrade_requests bereit.")
+        except Exception as e:
+            logger.warning(f"Migration upgrade_requests: {e}")
+
         # route_dogs: bestehende Routen allen Hunden des Users zuweisen
         try:
             existing = conn.execute("SELECT COUNT(*) FROM route_dogs").fetchone()[0]