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/web/discord.go

@@ -9,6 +9,7 @@ import (
 	"log"
 	"net/http"
 	"net/url"
+	"slices"
 	"strconv"
 	"strings"
 	"sync"
@@ -43,9 +44,9 @@ type DiscordConfig struct {
 	// RequiredRole, when non-empty, is a role ID a member must hold on top of
 	// guild membership. Empty by default: membership alone suffices.
 	RequiredRole string
-	// APIBBase is the Discord API root; configurable so tests run the whole
+	// APIBase is the Discord API root; configurable so tests run the whole
 	// flow against a local stub.
-	APIBBase string
+	APIBase string
 	// RedirectURI is the full public URL of the callback — Discord requires
 	// the exact string, so it is configured, never derived from headers.
 	RedirectURI string
@@ -59,7 +60,6 @@ type DiscordConfig struct {
 type oauthStates struct {
 	mu     sync.Mutex
 	expiry map[string]time.Time
-	order  []string // FIFO for eviction when the map is full
 }
 
 func newOAuthStates() *oauthStates {
@@ -75,18 +75,19 @@ func (s *oauthStates) put(state string, expires time.Time) {
 			delete(s.expiry, k)
 		}
 	}
+	// Evict the state closest to expiring when full, so a flood of starts
+	// cannot grow memory; at worst it invalidates an in-flight sign-in.
 	if len(s.expiry) >= maxStates {
-		// Evict from the front until under the cap. The front may already
-		// have been consumed by take (which removes from the map, not the
-		// order slice), so pop until the count actually drops.
-		for len(s.expiry) >= maxStates && len(s.order) > 0 {
-			oldest := s.order[0]
-			s.order = s.order[1:]
-			delete(s.expiry, oldest)
+		var oldest string
+		var oldestAt time.Time
+		for k, at := range s.expiry {
+			if oldest == "" || at.Before(oldestAt) {
+				oldest, oldestAt = k, at
+			}
 		}
+		delete(s.expiry, oldest)
 	}
 	s.expiry[state] = expires
-	s.order = append(s.order, state)
 }
 
 // take validates and consumes a state in one step: a state works exactly
@@ -107,7 +108,7 @@ func (s *oauthStates) take(state string) bool {
 func (h *Handler) discordStart(w http.ResponseWriter, r *http.Request) {
 	state := session.NewID()
 	h.states.put(state, time.Now().Add(oauthStateTTL))
-	u := h.discord.APIBBase + "/oauth2/authorize?" + url.Values{
+	u := h.discord.APIBase + "/oauth2/authorize?" + url.Values{
 		"client_id":     {h.discord.ClientID},
 		"redirect_uri":  {h.discord.RedirectURI},
 		"response_type": {"code"},
@@ -184,7 +185,7 @@ func (h *Handler) discordCallback(w http.ResponseWriter, r *http.Request) {
 	// The refusal is the same for a non-member and a member without the
 	// required role, and it names neither the guild nor its id: an outsider
 	// cannot tell whether the guild exists, let alone which one gates.
-	if !isMember || (h.discord.RequiredRole != "" && !contains(member.Roles, h.discord.RequiredRole)) {
+	if !isMember || (h.discord.RequiredRole != "" && !slices.Contains(member.Roles, h.discord.RequiredRole)) {
 		h.limiter.Fail(ip, time.Now())
 		h.renderLogin(w, http.StatusForbidden,
 			"This Discord account is not a member of this community.")
@@ -214,7 +215,7 @@ func (h *Handler) exchangeToken(ctx context.Context, code string) (discordToken,
 		"redirect_uri":  {h.discord.RedirectURI},
 	}
 	req, err := http.NewRequestWithContext(ctx, http.MethodPost,
-		h.discord.APIBBase+"/oauth2/token", strings.NewReader(form.Encode()))
+		h.discord.APIBase+"/oauth2/token", strings.NewReader(form.Encode()))
 	if err != nil {
 		return discordToken{}, err
 	}
@@ -241,7 +242,7 @@ func (h *Handler) exchangeToken(ctx context.Context, code string) (discordToken,
 // discordUserID fetches the signed-in user's id via the identify scope.
 func (h *Handler) discordUserID(ctx context.Context, accessToken string) (string, error) {
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet,
-		h.discord.APIBBase+"/users/@me", nil)
+		h.discord.APIBase+"/users/@me", nil)
 	if err != nil {
 		return "", err
 	}
@@ -276,7 +277,7 @@ type discordMember struct {
 // gate asks exactly the question it names. A 404 or 403 (not in the guild, or
 // the token lacks the scope) is a non-member, not an error.
 func (h *Handler) discordMember(ctx context.Context, accessToken, userID string) (discordMember, bool, error) {
-	u := h.discord.APIBBase + "/guilds/" + url.PathEscape(h.discord.GuildID) +
+	u := h.discord.APIBase + "/guilds/" + url.PathEscape(h.discord.GuildID) +
 		"/members/" + url.PathEscape(userID)
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, u, nil)
 	if err != nil {
@@ -305,12 +306,3 @@ func (h *Handler) discordMember(ctx context.Context, accessToken, userID string)
 type discordToken struct {
 	AccessToken string `json:"access_token"`
 }
-
-func contains(ss []string, want string) bool {
-	for _, s := range ss {
-		if s == want {
-			return true
-		}
-	}
-	return false
-}