feat(budget): auto-generate recurring entry instances per month

Adds schema migration v3 (recurrence_parent_id column + budget_recurrence_skipped
table). On every GET /api/v1/budget, the server checks all recurring originals
(is_recurring=1, no parent) and creates missing instances for the requested month
using the same day-of-month (clamped to the last day). Deleted instances are
recorded in budget_recurrence_skipped so they are not recreated on the next visit.
Generated instances are shown with a ↩ indicator in the transaction list.

Closes BL-05.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/db.js

@@ -278,6 +278,22 @@ const MIGRATIONS = [
       CREATE INDEX IF NOT EXISTS idx_calendar_external_id ON calendar_events(external_calendar_id);
     `,
   },
+  {
+    version: 3,
+    description: 'Wiederkehrende Budget-Einträge: parent-Referenz und Skip-Tabelle',
+    up: `
+      ALTER TABLE budget_entries ADD COLUMN recurrence_parent_id INTEGER
+        REFERENCES budget_entries(id) ON DELETE SET NULL;
+
+      CREATE TABLE IF NOT EXISTS budget_recurrence_skipped (
+        parent_id INTEGER NOT NULL REFERENCES budget_entries(id) ON DELETE CASCADE,
+        month     TEXT    NOT NULL,
+        PRIMARY KEY (parent_id, month)
+      );
+
+      CREATE INDEX IF NOT EXISTS idx_budget_parent ON budget_entries(recurrence_parent_id);
+    `,
+  },
 ];
 
 /**