feat(ui): clickable document viewer — open any cited document in the browser from chat chips and the sources table

README.md

@@ -72,6 +72,19 @@ uv run uvicorn app.main:app --reload
 > 📝 **After this, day-to-day is just: edit markdown → re-run the import.**
 > See [Updating the documents](#updating-the-documents) below.
 
+## Using the UI
+
+- **Chat** (`/`) — ask questions; answers stream in with **source chips**
+  that cite the exact documents used. Clicking a chip opens that document
+  **in a new tab**.
+- **Document viewer** (`/document.html?source=…&path=…`) — the full text of
+  any indexed document, served from the database (no filesystem access):
+  markdown is rendered, every other format (`yaml`, `json`, `py`, `txt`, …)
+  is shown as escaped monospace text. Unknown documents get a designed
+  not-found state with a link back to the index.
+- **Sources** (`/sources.html`) — the indexed document list; the *Path*
+  column links each document to the viewer in a new tab.
+
 ## Updating the documents
 
 **This is the workflow you'll use most.** The knowledge base is refreshed by