feat(stats): optimize dashboard data loading with internal ui endpoints

CHANGELOG.md

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+# Changelog
+
+## Unreleased
+
+### Changed
+
+- Optimized the HTML dashboard data flow by splitting board loading and task detail loading.
+- Kept the public `/api/tasks/*` API unchanged.
+- Added internal `/ui_data/*` routes for lightweight board data and on-demand task details.
+- Updated the `/stats` page to load a lightweight board model first and fetch full task details only for the selected task.
+- Hid internal UI routes from Swagger / OpenAPI documentation.
+- Updated tests and project documentation to match the new dashboard behavior.
+
+### Why
+
+- The previous dashboard rendered the full task payload for every card directly into the HTML.
+- This caused unnecessary data transfer and made the page size grow linearly with the number of tasks.
+- The change reduces the initial payload and loads detailed data only when it is actually needed.
+
+### Effect
+
+- The `/stats` HTML response became nearly constant in size instead of growing with the full board.
+- Embedded per-card task payloads were removed from the DOM.
+- The dashboard now transfers less data on initial load and keeps full task details as a separate targeted request.
+- Measured impact on a 1000-task dataset:
+  - `/stats` HTML reduced from `632,377 B` to `11,413 B`
+  - embedded task payload reduced from `250,800 B` to `0 B`
+  - full board data moved into a lightweight JSON response (`162,656 B`)
+  - selected task details are loaded separately in a small response (`179 B`)