n0-search-pdf-page-ranges

From the team thread about n0's search:

> when we search documents, for PDFs does it give page numbers? is n0 hiding
> any info from the raw agent search response?

It doesn't: a search result says which document matched, never where in it.
Agent Search can tell us which source pages a matching passage came from,
and we want that on every search result n0 hands out:

> lets do that so we can see the page numbers

- In the backend's search API and the MCP `search` tool, each result carries
  the page range of the passage that matched best, as `page_start` and
  `page_end` (page numbers). When there is no page information, as for most
  documents that are not PDFs, both are null.
- In the web app, the search result card shows it: `Page 7` for a single
  page, `Pages 7–9` for a range, and nothing when there are no pages.
- Search still returns one result per document. A big PDF can match a query
  in many passages: that must not crowd the other matching documents out of
  the results, but one search must not page through every passage of a huge
  one to find them either.

## Environment

PostgreSQL on localhost:5432 and the local database endpoint the app's
driver uses are running when you start, and the dev env files point at the
dev database.