chore: init monorepo with GPL-3.0 license, docs, backend skeleton, frontend wiring

indexium-backend/migrations/20260908000000_webhook_deliveries.sql

@@ -0,0 +1,8 @@
+CREATE TABLE IF NOT EXISTS webhook_deliveries (
+    delivery_id VARCHAR(64) PRIMARY KEY,
+    event VARCHAR(32) NOT NULL,
+    action VARCHAR(32),
+    repo_id BIGINT,
+    payload JSONB,
+    processed_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);