feat(110): statistics summary

Phase 110 complete:
- add Statistics -> Podsumowanie page
- add monthly order count and value charts per integration plus total
- use Chart.js with table fallback and 04-2026 default history start
- update PAUL and DOCS technical documentation

.paul/codebase/db_schema.md

@@ -850,3 +850,14 @@ Default keys: `cron_run_on_web`, `cron_web_limit`, `gs1_api_login`, `gs1_prefix`
 | Audit via JSON | `payload_json` snapshots in orders, shipments, receipts |
 | Migrations | `database/migrations/YYYYMMDD_NNNNNN_description.sql` |
 | Deferred indexes | `idx_order_addresses_order_type`, `idx_shipment_packages_order_delivery` — apply at >50k orders |
+
+## Reporting Usage
+
+**Statistics Summary (`/statistics/summary`)** — no dedicated reporting tables.
+- Reads existing `orders` rows and groups by month using the same effective order date used by `/statistics/orders`.
+- Default summary history starts at April 2026 (`2026-04-01`), even if older rows exist.
+- Splits series by channel key: Allegro as one series and each shopPRO integration by `orders.integration_id`.
+- Uses `integrations.name` only for display labels when available.
+- Filters by selected status groups through `order_status_groups` and `order_statuses`.
+- Uses existing gross amount columns via `OrdersStatisticsRepository::grossAmountSql()`.
+- No schema migration was introduced for Phase 110.