Require secrets via headers only: drop ?key= and ?password= query fallbacks (#137, #141)

Query strings leak into Traefik access logs, browser history, and Referer
headers. Admin key is now accepted only via X-Admin-Key; paste and can
passwords only via X-Paste-Password (or the POST unlock form). Tests
updated; new negative cases assert 401 for the query paths.

internal/api/cans.go

@@ -206,9 +206,6 @@ func (a *apiServer) handleGetCan(w http.ResponseWriter, r *http.Request) {
 	}
 	if can.PasswordHash.Valid {
 		pw := r.Header.Get("X-Paste-Password")
-		if pw == "" {
-			pw = r.URL.Query().Get("password")
-		}
 		if pw == "" || !store.CheckPassword(can.PasswordHash.String, pw) {
 			writeErr(w, 401, "password required")
 			return
@@ -256,9 +253,6 @@ func (a *apiServer) handleCanItem(w http.ResponseWriter, r *http.Request) {
 	can, _ := a.store.GetCan(row.CanID.String)
 	if can != nil && can.PasswordHash.Valid {
 		pw := r.Header.Get("X-Paste-Password")
-		if pw == "" {
-			pw = r.URL.Query().Get("password")
-		}
 		if pw == "" || !store.CheckPassword(can.PasswordHash.String, pw) {
 			// fall back to the browser's unlock cookie for this can
 			c, cerr := r.Cookie("pw_" + can.ID)