chore(ci): add govulncheck, staticcheck, go vet, gofumpt gates

* Add lint/format checks to the Gitea release workflow and Makefile
  (targets: vet, fmt, fmt-check, staticcheck, govulncheck, check)
* Switch .golangci.json formatter from gofmt to gofumpt (extra.group-params)
* Bump golang.org/x/text 0.37.0 -> 0.39.0 for GO-2026-5970; re-vendor
* Fix staticcheck S1011 in pkg/diff; drop unused pgsql writer helpers
* Fix gocritic unnamedResult (pkg/diff) and rangeValCopy (pkg/pgsql)
* Apply gofumpt + goimports formatting across the tree

.golangci.json

@@ -1,7 +1,7 @@
 {
   "formatters": {
     "enable": [
-      "gofmt",
+      "gofumpt",
       "goimports"
     ],
     "exclusions": {
@@ -13,8 +13,10 @@
       ]
     },
     "settings": {
-      "gofmt": {
-        "simplify": true
+      "gofumpt": {
+        "extra": {
+          "group-params": true
+        }
       },
       "goimports": {
         "local-prefixes": [