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.

.golangci.yml

@@ -1,18 +1,17 @@
+version: "2"
+
 run:
   timeout: 5m
   tests: true
 
 linters:
-  disable-all: true
+  default: none
   enable:
     - errcheck
-    - gosimple
     - govet
     - ineffassign
     - staticcheck
     - unused
-    - gofumpt
-    - goimports
     - misspell
     - revive
     - bodyclose
@@ -21,22 +20,29 @@ linters:
     - prealloc
     - unconvert
     - unparam
-
-linters-settings:
-  goimports:
-    local-prefixes: gitea.dcglab.co.uk/steve/restic-manager
-  revive:
+  settings:
+    revive:
+      rules:
+        - name: exported
+          arguments: ["disableStutteringCheck"]
+    misspell:
+      locale: US
+  exclusions:
     rules:
-      - name: exported
-        arguments: ["disableStutteringCheck"]
-  misspell:
-    locale: US
+      - path: _test\.go
+        linters:
+          - errcheck
+          - unparam
+
+formatters:
+  enable:
+    - gofumpt
+    - goimports
+  settings:
+    goimports:
+      local-prefixes:
+        - gitea.dcglab.co.uk/steve/restic-manager
 
 issues:
-  exclude-rules:
-    - path: _test\.go
-      linters:
-        - errcheck
-        - unparam
   max-issues-per-linter: 0
   max-same-issues: 0