refactor(web): harden oauth state store and session writes after review

- Drop the FIFO from oauthStates: consumed states left entries behind, so
  an unrate-limited start/cancel cycle grew the slice without bound.
  Evict by oldest expiry instead — the map alone now bounds memory.
- CreateSession runs INSERT + expiry sweep in one transaction.
- slices.Contains replaces a hand-rolled contains; APIBase typo fixed.
- Stale comments and test paths updated; login hover uses --ember-ink.

backend/internal/latest/poller.go

@@ -30,8 +30,8 @@ type Fetcher interface {
 // cannot shorten anyone's cooldown; it only makes the poller wake up and find
 // nothing due more often.
 type Poller struct {
-	Store    *store.Store
-	Fetch    Fetcher
+	Store *store.Store
+	Fetch Fetcher
 	// BrowserFetch handles sites behind a JavaScript challenge that Fetch
 	// cannot clear. Nil disables those sites entirely rather than falling back
 	// to Fetch, which would only ever retrieve a challenge page.