P2R-02 slice 4: Repo tab — connection / bandwidth / maintenance

Three independent forms on /hosts/{id}/repo so saving one section
doesn't disturb the others:

* Connection: edits repo URL, username, password (pre-filled from
  the redacted GET /api/hosts/{id}/repo-credentials view; password
  field shows masked stored-creds placeholder; blank password = keep
  existing). On save, encrypts and pushes config.update to a
  connected agent.
* Bandwidth: host-wide upload/download caps (KB/s; blank = no cap)
  written via store.SetHostBandwidth. New REST endpoint
  PUT /api/hosts/{id}/bandwidth for JSON callers.
* Maintenance: forget/prune/check cadences + check subset %, with
  per-row enabled toggles. Reuses cronParser for validation;
  auto-seeds the row if a host pre-dates the migration.

Right-rail surfaces repo size, snapshot count, snapshots-by-tag
breakdown (counted from existing snapshot tag rows), and an
'untagged snapshots are left alone' note.

Danger-zone re-init button is rendered but disabled with a hint
pointing at P2R-09 (real implementation lands there).

Validation re-renders the page with the relevant form's banner and
all other section state intact. Successful saves redirect with a
?saved=<section> query param so the page surfaces a small ✓ saved
indicator on the relevant form.

ci.yml: bump golangci-lint-action v6→v7 (separate change picked up
in this commit).

.gitea/workflows/ci.yml

@@ -34,7 +34,7 @@ jobs:
         with:
           go-version: ${{ env.GO_VERSION }}
           cache: true
-      - uses: golangci/golangci-lint-action@v6
+      - uses: golangci/golangci-lint-action@v7
         with:
           # v1.61 was built against Go 1.23 and refuses to load a
           # config that targets a newer toolchain — go.mod is on 1.25.