ci: migrate .golangci.yml to v2 schema + only-new-issues gate

The bump from golangci-lint-action@v6 → v7 (which downloads the v2.x
binary) was blocking CI lint with 'unsupported version of the
configuration: ""' because .golangci.yml was still in the v1 schema.

Migrate the config to v2:
* version: "2" prelude
* disable-all → default: none
* linters-settings → linters.settings
* gofumpt + goimports move into formatters.enable + formatters.settings
* exclude-rules move into linters.exclusions.rules
* gosimple drops (folded into staticcheck in v2)

Fix the four lint hits in the new P2R-02 code:
* host_bandwidth.go: convert hostBandwidthRequest directly to
  hostBandwidthView via type conversion (S1016)
* ui_repo.go: drop unparam savedSection + status arguments from
  renderRepoPage (always "" / always 422 — split GET render from
  validation-fail render)
* ui_schedules.go: gofumpt formatting on the scheduleEditPage struct

Add only-new-issues: true to the lint job. The repo carries ~90
pre-existing findings (gofumpt drift × 31, misspell × 25, missing
godoc × 10, bodyclose × 6, errcheck × 12, …) accumulated before
lint was actually wired into CI. Without this gate, every PR would
fail on baseline noise instead of its own changes.

Track the cleanup as X-06 in tasks.md so the gate is temporary.

internal/server/http/host_bandwidth.go

@@ -58,8 +58,5 @@ func (s *Server) handleUpdateHostBandwidth(w stdhttp.ResponseWriter, r *stdhttp.
 		writeJSONError(w, stdhttp.StatusInternalServerError, "internal", err.Error())
 		return
 	}
-	writeJSON(w, stdhttp.StatusOK, hostBandwidthView{
-		BandwidthUpKBps:   req.BandwidthUpKBps,
-		BandwidthDownKBps: req.BandwidthDownKBps,
-	})
+	writeJSON(w, stdhttp.StatusOK, hostBandwidthView(req))
 }