Add invite routes to backend and frontend; implement routing and UI components for invitation management

backend/create-invitation-table.sql

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+-- SQL Script: Invitation-Tabelle erstellen
+-- Speichert gesendete Einladungen mit Spam-Schutz
+-- Ausführen mit: mysql -u root -p stechuhr < create-invitation-table.sql
+
+USE stechuhr;
+
+-- Erstelle Invitation-Tabelle falls nicht vorhanden
+CREATE TABLE IF NOT EXISTS invitation (
+  id BIGINT AUTO_INCREMENT PRIMARY KEY,
+  inviter_user_id BIGINT NOT NULL,
+  email VARCHAR(255) NOT NULL,
+  token VARCHAR(255) NOT NULL UNIQUE,
+  created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  expires_at DATETIME NOT NULL,
+  status VARCHAR(20) NOT NULL DEFAULT 'pending',
+  
+  -- Foreign Keys
+  CONSTRAINT fk_invitation_user FOREIGN KEY (inviter_user_id) 
+    REFERENCES user(id) ON DELETE CASCADE,
+  
+  -- Indices für Performance
+  INDEX idx_invitation_token (token),
+  INDEX idx_invitation_email_created (email, created_at),
+  INDEX idx_invitation_status (status)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+
+SELECT 'Invitation-Tabelle erfolgreich erstellt oder bereits vorhanden.' AS status;
+