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/agent/runner/runner.go

@@ -99,13 +99,13 @@ func (r *Runner) RunBackup(ctx context.Context, jobID string, paths, excludes, t
 			}
 			lastProgress = time.Now()
 			progEnv, _ := api.Marshal(api.MsgJobProgress, jobID, api.JobProgressPayload{
-				JobID:       jobID,
-				PercentDone: status.PercentDone,
-				FilesDone:   status.FilesDone,
-				TotalFiles:  status.TotalFiles,
-				BytesDone:   status.BytesDone,
-				TotalBytes:  status.TotalBytes,
-				ETASeconds:  status.SecondsRem,
+				JobID:         jobID,
+				PercentDone:   status.PercentDone,
+				FilesDone:     status.FilesDone,
+				TotalFiles:    status.TotalFiles,
+				BytesDone:     status.BytesDone,
+				TotalBytes:    status.TotalBytes,
+				ETASeconds:    status.SecondsRem,
 				ThroughputBps: throughput(status.BytesDone, status.SecondsElapsed),
 			})
 			_ = r.tx.Send(progEnv)