M6: CSV-Import mit Vorschau, Dublettenpruefung und Audit-Trail

csvupload.php verarbeitete CSV-Zeilen bisher sofort beim Upload, ohne
Vorschau, ohne Zugriffskontrolle (nur CSRF) und schrieb ausschliesslich in
die global unscoped kl_Einzahlungen-Tabelle.

- Neue Tabellen payment_import_batches/payment_import_rows protokollieren
  jeden Import: Datei, Pruefsumme, jede Zeile mit Rohwerten, erkanntem
  Mitglied, Status und erzeugter Ledger-Zeile.
- Zweistufiger Ablauf: Hochladen zeigt nur eine Vorschau (matched/
  duplicate/unmatched/invalid), erst "Import bestaetigen" bucht.
- Zuordnung per paypal_name oder display_name (tenant-scoped), Dubletten-
  pruefung gegen bestehende nicht-stornierte Ledger-Zahlungen.
- Buchung folgt dem etablierten Zweig-Muster: Default-Mandant per
  Dual-Write nach kl_Einzahlungen plus Spiegelung, alle anderen Mandanten
  direkt ueber ledger_record_payment().
- Zugriffskontrolle ergaenzt (owner/admin/treasurer + Legacy-Fallback).
- Live getestet: Treffer, unbekannter Name, ungueltiger Betrag korrekt
  klassifiziert; Import bestaetigt mit korrektem Dual-Write; erneuter
  Upload derselben Datei erkennt die Zeile korrekt als Dublette.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

database/migrations/0008_saas_payment_imports.sql

@@ -0,0 +1,61 @@
+CREATE TABLE IF NOT EXISTS payment_import_batches (
+  id INT AUTO_INCREMENT PRIMARY KEY,
+  tenant_id INT NOT NULL,
+  original_filename VARCHAR(255) NOT NULL,
+  checksum CHAR(64) NOT NULL,
+  status VARCHAR(30) NOT NULL DEFAULT 'previewed',
+  created_by_user_id INT NULL,
+  created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  committed_at DATETIME NULL,
+  KEY idx_payment_import_batches_tenant (tenant_id, created_at),
+  CONSTRAINT fk_payment_import_batches_tenant
+    FOREIGN KEY (tenant_id) REFERENCES tenants(id)
+    ON DELETE CASCADE,
+  CONSTRAINT fk_payment_import_batches_created_by_user
+    FOREIGN KEY (created_by_user_id) REFERENCES users(id)
+    ON DELETE SET NULL
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+
+CREATE TABLE IF NOT EXISTS payment_import_rows (
+  id INT AUTO_INCREMENT PRIMARY KEY,
+  batch_id INT NOT NULL,
+  row_num INT NOT NULL,
+  participant_id INT NULL,
+  raw_name VARCHAR(255) NOT NULL,
+  amount_cents INT NOT NULL,
+  booked_at DATETIME NOT NULL,
+  status VARCHAR(30) NOT NULL,
+  raw_json TEXT NULL,
+  ledger_entry_id INT NULL,
+  created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  KEY idx_payment_import_rows_batch (batch_id),
+  KEY idx_payment_import_rows_participant (participant_id),
+  CONSTRAINT fk_payment_import_rows_batch
+    FOREIGN KEY (batch_id) REFERENCES payment_import_batches(id)
+    ON DELETE CASCADE,
+  CONSTRAINT fk_payment_import_rows_participant
+    FOREIGN KEY (participant_id) REFERENCES participants(id)
+    ON DELETE SET NULL,
+  CONSTRAINT fk_payment_import_rows_ledger_entry
+    FOREIGN KEY (ledger_entry_id) REFERENCES ledger_entries(id)
+    ON DELETE SET NULL
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+
+SET @schema_name = DATABASE();
+
+SET @sql = (
+  SELECT IF(
+    EXISTS (
+      SELECT 1
+      FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS
+      WHERE TABLE_SCHEMA = @schema_name
+        AND TABLE_NAME = 'ledger_entries'
+        AND CONSTRAINT_NAME = 'fk_ledger_entries_import_batch'
+    ),
+    'SELECT 1',
+    'ALTER TABLE ledger_entries ADD CONSTRAINT fk_ledger_entries_import_batch FOREIGN KEY (import_batch_id) REFERENCES payment_import_batches(id) ON DELETE SET NULL'
+  )
+);
+PREPARE stmt FROM @sql;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;