auth: add RequireSession chi-compatible middleware

Implements HTTP middleware that enforces session authentication by validating
session cookies and making claims available via ClaimsFromContext. Rejects
requests without valid session cookies with 401 Unauthorized.

Tested via four test cases:
- Missing session cookie rejection
- Valid cookie acceptance with claims extraction
- Expired cookie rejection
- Tampered cookie rejection

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

backend/internal/auth/middleware.go

@@ -0,0 +1,38 @@
+package auth
+
+import (
+	"context"
+	"net/http"
+)
+
+type contextKey int
+
+const claimsContextKey contextKey = iota
+
+// RequireSession returns middleware that rejects a request with 401 unless
+// it carries a valid SessionCookieName cookie signed with secret, and
+// otherwise makes the session's Claims available via ClaimsFromContext.
+func RequireSession(secret []byte) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			cookie, err := r.Cookie(SessionCookieName)
+			if err != nil {
+				http.Error(w, "not authenticated", http.StatusUnauthorized)
+				return
+			}
+			claims, err := ParseSessionCookie(cookie, secret)
+			if err != nil {
+				http.Error(w, "not authenticated", http.StatusUnauthorized)
+				return
+			}
+			next.ServeHTTP(w, r.WithContext(context.WithValue(r.Context(), claimsContextKey, claims)))
+		})
+	}
+}
+
+// ClaimsFromContext returns the authenticated user's Claims, as populated by
+// RequireSession.
+func ClaimsFromContext(ctx context.Context) (Claims, bool) {
+	c, ok := ctx.Value(claimsContextKey).(Claims)
+	return c, ok
+}