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/jobhub.go

@@ -33,7 +33,7 @@ func NewJobHub() *JobHub {
 // the hub's set (so concurrent Broadcasts will reach it), but no
 // pump goroutine runs yet. The caller can prime the channel via Send
 // — useful for late-subscriber catch-up — and then call Run to start
-// the pump. Run blocks until ctx is cancelled or conn dies, and
+// the pump. Run blocks until ctx is canceled or conn dies, and
 // unregisters on return.
 type Subscriber struct {
 	hub   *JobHub
@@ -73,7 +73,7 @@ func (s *Subscriber) Send(env api.Envelope) {
 }
 
 // Run pumps messages from the subscriber's channel onto conn until
-// ctx is cancelled or conn dies. Unregisters on return. Caller is
+// ctx is canceled or conn dies. Unregisters on return. Caller is
 // expected to invoke this from the goroutine that owns conn.
 func (s *Subscriber) Run(ctx context.Context, conn *Conn) {
 	defer s.unregister()