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/ws/hub_test.go

@@ -57,13 +57,18 @@ func TestWSHelloAndHeartbeat(t *testing.T) {
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
 
-	c, _, err := websocket.Dial(ctx, url, &websocket.DialOptions{
+	c, res, err := websocket.Dial(ctx, url, &websocket.DialOptions{
 		HTTPHeader: stdhttp.Header{"Authorization": []string{"Bearer " + token}},
 	})
 	if err != nil {
 		t.Fatalf("dial: %v", err)
 	}
 	defer c.CloseNow()
+	defer func() {
+		if res != nil && res.Body != nil {
+			_ = res.Body.Close()
+		}
+	}()
 
 	// Send hello.
 	hello := api.HelloPayload{
@@ -125,13 +130,18 @@ func TestWSRejectsOldProtocol(t *testing.T) {
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
 
-	c, _, err := websocket.Dial(ctx, url, &websocket.DialOptions{
+	c, res, err := websocket.Dial(ctx, url, &websocket.DialOptions{
 		HTTPHeader: stdhttp.Header{"Authorization": []string{"Bearer " + token}},
 	})
 	if err != nil {
 		t.Fatalf("dial: %v", err)
 	}
 	defer c.CloseNow()
+	defer func() {
+		if res != nil && res.Body != nil {
+			_ = res.Body.Close()
+		}
+	}()
 
 	hello := api.HelloPayload{ProtocolVersion: 0} // below minimum
 	env, _ := api.Marshal(api.MsgHello, "", hello)
@@ -170,6 +180,13 @@ func TestWSRejectsBadToken(t *testing.T) {
 	_, res, err := websocket.Dial(ctx, url, &websocket.DialOptions{
 		HTTPHeader: stdhttp.Header{"Authorization": []string{"Bearer wrong"}},
 	})
+	if res != nil {
+		defer func() {
+			if res != nil && res.Body != nil {
+				_ = res.Body.Close()
+			}
+		}()
+	}
 	if err == nil {
 		t.Fatal("dial should fail")
 	}