archived repositories and topics #23

At 65 repos, discovery and done-signaling matter. Archive: a repo setting making it read-only (pushes and issue/MR writes refused, browsing untouched) with a badge. Topics: free-form tags on repos, shown on index/owner pages and filterable; feeds the design work in #10 and search in #7.

Backend shipped in d298215: repo archive/unarchive (pushes refused in the SSH transport; issue/MR/status writes refused with exit 4; browsing, cloning, settings, access, transfer, delete unaffected) and repo topics list/add/remove (repo_topics table, [a-z0-9-]{1,35}, max 20, admin-only edits). CLI passthrough wired; e2e in e2e/archive_test.go. Web badge and topic chips land after the #10 design pass to avoid template collisions.

Complete: backend in d298215, web rendering (archived chip, topic chips) in 027a724 as part of the design pass. Topic filtering on the index can ride along with search (#7).