Record the default branch summary when a push lands

Why

Closes #130. The landing card showed the default branch tip (SHA, subject, author, time) and the language the repository is mostly made of by reading git on every home page render: symbolic-ref, rev-parse, log -1 and ls-tree -r per repository. Those facts change only when a push moves the branch.

What changed

• repos gains head_sha, head_subject, head_author, head_when, language and lang_class; index.Entry returns them with the rest of the row.
• Store.RefreshHead recomputes the summary. It compares the recorded tip with the branch tip first, so a metadata or tag push costs one rev-parse and no tree walk. It runs from post-receive (every push), from the rebuild (IndexRepo / vole admin index rebuild), and after a default branch change in settings.
• The language mapping moved to internal/lang, shared by the web pages and the summary; languageFor and fileClass are now one table.
• The home card reads head, language, counts, ACL and newest event time from the index. The home page starts no git process. The all-git card builder stays as the fallback for a Web without an index.

Measurement

Load harness, concurrency 4, three repositories:

git-children sampled during the run: 0.

Tests

• TestRefreshHeadRecordsTheDefaultBranch: tip, subject, author, time, language and class from a pushed main branch.
• TestRefreshHeadClearsADeletedBranch.
• TestIndexRepoRestoresTheHeadSummary.
• Existing home card and top-language tests now exercise the hook’s summary; the language table’s tests moved to internal/lang.
• Full -race suite, vet, gofmt, link check.

Follow-up

internal/lang is the one place the display name and CSS class live now; a future language addition is one switch.