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/store/sources_test.go

@@ -177,7 +177,7 @@ func TestPendingRunQueue(t *testing.T) {
 
 	now := time.Now().UTC()
 	if err := s.EnqueuePendingRun(ctx, &PendingRun{
-		ID: "01HPEND00000000000000001",
+		ID:         "01HPEND00000000000000001",
 		ScheduleID: schedID, SourceGroupID: gid, HostID: hostID,
 		NextAttemptAt: now.Add(-time.Second), // already due
 		ScheduledAt:   now.Add(-time.Minute),