lint: drive baseline to zero, drop only-new-issues gate

Cleanup pass over the repo so CI can enforce lint going forward
without the only-new-issues escape hatch:

* gofumpt -w across the tree (31 hits, all formatting)
* misspell --fix (25 hits, US-locale spelling) — but reverted on
  api.JobCancelled = "cancelled" since that literal is the wire +
  DB CHECK constraint value, plus matched the case in store/fleet.go
  back to "cancelled" and added //nolint:misspell on both for the
  next time someone reaches for the auto-fix
* Wrap every `defer rows.Close()` / `defer stmt.Close()` /
  `defer res.Body.Close()` in `defer func() { _ = .Close() }()`
  to satisfy errcheck without losing the close itself
* websocket.Dial callers (1 prod, 4 tests) now capture + close the
  upgrade response Body — coder/websocket can return res with a nil
  Body on success, so the test deferred-closes guard against that
* Annotate the two genuine-by-design nilerr cases with //nolint
  comments explaining why nil-on-error is the contract (cookie
  missing = no session; ctx cancelled mid-backoff = clean shutdown)
* Add brief godoc on the 10 exported const groups + types that
  revive flagged (api.HostOS/HostArch/JobKind/JobStatus/LogStream/
  ErrorCode, restic.EventKind, store.Role, web.FS)
* Drop the unused (*Server).userByID method
* Inline the unparam baseView(active) — every UI page is under
  the dashboard primary nav today

Result: `golangci-lint run ./...` reports 0 issues. CI lint job
no longer needs only-new-issues: true; X-06 follow-up entry in
tasks.md removed.

internal/server/http/auth_test.go

@@ -36,7 +36,7 @@ func newTestServer(t *testing.T, withBootstrapToken bool) (*Server, string) {
 	aead, _ := crypto.NewAEAD(key)
 
 	deps := Deps{
-		Cfg: config.Config{Listen: ":0", DataDir: dir, SecretKeyFile: keyPath},
+		Cfg:   config.Config{Listen: ":0", DataDir: dir, SecretKeyFile: keyPath},
 		Store: st,
 		AEAD:  aead,
 	}
@@ -125,7 +125,9 @@ func TestLoginAndLogout(t *testing.T) {
 	bs, _ := json.Marshal(bootstrapRequest{
 		Token: "test-token", Username: "alice", Password: "averylongpassword",
 	})
-	stdhttp.Post(url+"/api/bootstrap", "application/json", bytes.NewReader(bs)) //nolint:errcheck
+	if bsRes, err := stdhttp.Post(url+"/api/bootstrap", "application/json", bytes.NewReader(bs)); err == nil {
+		_ = bsRes.Body.Close()
+	}
 
 	// Login.
 	body, _ := json.Marshal(loginRequest{Username: "alice", Password: "averylongpassword"})