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_flow_test.go

@@ -294,7 +294,8 @@ func TestCanItemCookieParity(t *testing.T) {
 	}
 
 	// item id from API (with password query)
-	req = httptest.NewRequest("GET", "/api/cans/"+created.ID+"?password=pw123", nil)
+	req = httptest.NewRequest("GET", "/api/cans/"+created.ID, nil)
+	req.Header.Set("X-Paste-Password", "pw123")
 	rec = httptest.NewRecorder()
 	h.ServeHTTP(rec, req)
 	var can struct {