M4 Ledger-Migration fuer Legacy-Buchungen starten

- ledger_entries als tenant-sichere Buchungstabelle ergaenzen
- Legacy-Einzahlungen und Kaffeeverbrauch idempotent spiegeln
- M4-Paritaetscheck und Datenmigrationsdoku ergaenzen

database/migrations/0006_saas_ledger_entries.sql

@@ -0,0 +1,38 @@
+CREATE TABLE IF NOT EXISTS ledger_entries (
+  id INT AUTO_INCREMENT PRIMARY KEY,
+  tenant_id INT NOT NULL,
+  participant_id INT NOT NULL,
+  type VARCHAR(30) NOT NULL,
+  amount_cents INT NOT NULL,
+  marks_count INT NULL,
+  unit_price_cents INT NULL,
+  booked_at DATETIME NOT NULL,
+  source VARCHAR(40) NOT NULL DEFAULT 'legacy',
+  note VARCHAR(1000) NULL,
+  created_by_user_id INT NULL,
+  import_batch_id INT NULL,
+  legacy_table VARCHAR(100) NULL,
+  legacy_id INT NULL,
+  voided_at DATETIME NULL,
+  reversal_of_entry_id INT NULL,
+  created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+  UNIQUE KEY uq_ledger_entries_tenant_legacy (tenant_id, legacy_table, legacy_id),
+  KEY idx_ledger_entries_tenant_booked_at (tenant_id, booked_at),
+  KEY idx_ledger_entries_participant_booked_at (participant_id, booked_at),
+  KEY idx_ledger_entries_tenant_type (tenant_id, type),
+  KEY idx_ledger_entries_reversal (reversal_of_entry_id),
+  KEY idx_ledger_entries_created_by_user (created_by_user_id),
+  CONSTRAINT fk_ledger_entries_tenant
+    FOREIGN KEY (tenant_id) REFERENCES tenants(id)
+    ON DELETE CASCADE,
+  CONSTRAINT fk_ledger_entries_participant
+    FOREIGN KEY (participant_id) REFERENCES participants(id)
+    ON DELETE CASCADE,
+  CONSTRAINT fk_ledger_entries_created_by_user
+    FOREIGN KEY (created_by_user_id) REFERENCES users(id)
+    ON DELETE SET NULL,
+  CONSTRAINT fk_ledger_entries_reversal
+    FOREIGN KEY (reversal_of_entry_id) REFERENCES ledger_entries(id)
+    ON DELETE SET NULL
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;