Add CSV export of scan results

The Review tab gets an Export CSV menu: "This view" exports the current
status filter and search, "Entire scan" exports every scanned slave file.

Each row carries status, match type, a recommendation, slave/master paths,
the link target, the match reason, sizes, codec details for both sides,
the quality verdict, notes, errors and timestamps. Rows are streamed in
pages of 1000 so a large export never holds the database connection a
running scan needs. The file starts with a UTF-8 BOM for Excel, and cells
a spreadsheet would evaluate as formulas are prefixed with an apostrophe.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/server.go

@@ -83,6 +83,7 @@ func (s *Server) Handler() http.Handler {
 				r.Get("/job", s.handleJob)
 				r.Post("/job/cancel", s.handleCancelJob)
 				r.Get("/matches", s.handleMatches)
+				r.Get("/export.csv", s.handleExport)
 				r.Post("/link-identical", s.handleLinkIdentical)
 				r.Get("/tree", s.handleTree)
 				r.Post("/build", s.handleBuild)